Lotta verbage here, I'm gonna cut to the chase of "two things".
Thing One: it minimizes how much a robber can take. We already agree. I say that's the ONLY thing it does.
Thing Two: it makes robbery "less probable" --- well, no. It can't. (a) Said robber has no idea how much is in there, (b) said robber MAY know that's probably the policy, but then the drawer is hardly his only resource once he's in the act. The first thing he went for was not the drawer but the safe. He could have also robbed people individually. It ain't like the contents of a cash drawer are all there is, and even if there's only 20 bucks in there -- if he gets it, that's 20 bucks he didn't have when he walked in and pulled a gun. It's not like armed robbery is an exact science with pie-chart thresholds defining how much money is worth pulling a job.
Most studies on the matter the world over disagree with you on point two:
"This paper describes a research project conducted in five states to provide more detailed information on convenience store robberies and to identify ways in which these robberies might be reduced.
[...]
One of the earliest studies that set the foundation for subsequent studies of convenience stores was conducted by Crow and Bull (1975) in conjunction with the National Institute of Law Enforcement and Criminal Justice (NILECJ) and the Southland Corporation, then the largest owner of western 7-11 convenience stores. Crow and Bull
surveyed 349 convicted armed robbers and asked them to identify store characteristics that could influence their decision to rob a store. The robbers' responses were then used to develop a ranking of stores according to their attractiveness to robbery. Based on this scale, Crow and Bull developed several robbery prevention measures. The prevention measures consisted of: strategic placing of signs announcing a low amount of cash on hand in the store: moving cash drop boxes to make them more conspicuous: improving lighting in the parking lot: keeping the store clean: enhancing employee alertness: and greeting each person who came in.
[...]
"Swanson (1986) conducted a three-part analysis of the convenience store robbery problem. Swanson first interviewed 65 convenience store robbers incarcerated
in Florida and asked them to rank in order the most desirable characteristics in choosing a store to rob. The five most desirable characteristics the robbers ranked were: (1)
remote area (45%); (3) no customers (32%): (3) one clerk on duty (32%); (4) easy access/get away (25%); (5) lots of cash (25%).
As suggested by Bentham's (1979)
utility theory, Swanson's research suggests that convenience store robbers weigh the costs and benefits before committing a robbery." ~
http://www.ncjrs.gov/pdffiles1/nij/grants/173772.pdf
"
Decision making practice on who to rob (choice of targets/victims)
According to Conklin (1972), Thio (1998:326),
once the decision to rob has been made, the next step for the robbers is to select a certain target. Choice of would-be-victim or target is therefore a top priority in robbery escapades. Choices are neither ordinarily, nor haphazardly made. They are often based on certain criteria, and evaluated on cost-benefit analysis based on certain factors such as lucrative establishments and lower risk of arrest (see also Conklin, 1972; Thio, 1998:326).
The findings in this study confirm the importance of decision making practice on the choice of who to rob. We found a total of 70.6% of our 68 sample who responded to the variable accepted making such decision.
[...]
These findings revealed that decisions on the choice of targets are not made on a vacuum and they find support with previous studies on targets or victims’ selection (see Conklin, 1972; Thio, 1998) According to these authors,
three factors are crucial to the gang of armed robbers or the lone-armed robber while making decision bothering on the selection of the targets. These are lucrative, establishment, and risk assessment." ~
Society for Research and Academic Excellence
"Conducted as a follow-up to a 1985 survey, the Athena Research Corporation (Athena) study concluded that robbers target stores with easy access to cash and easy escape routes without regard to alarms or surveillance technology. According to the survey, the number one concern for culprits in planning a robbery is how to escape.
Their second priority is the amount of money they will get.
[...]
MOTIVATIONS. Fifty-five percent of those surveyed in 1995 said they
would need to pocket at least $200 to be motivated enough to steal. Forty-five percent of respondents said that they would be willing to rob a convenience store because they expect to get at least $200 from these establishments.
[...]
Effective.
To determine what would make a robber more reluctant to rob a store, respondents were asked what was most important in deciding which convenience store to rob. In 1985, the most important factor from among eleven was money, and a good escape route was second. In 1995, an escape route was first and money second, due at least in part to stores reducing the amount of money kept on hand. One robber summed up the major findings of the survey when asked what had ever kept him from robbing. He said, "A bad escape route, and the cashier drops the cash too often." ~
Crimes of convenience a study of what motivates robbers finds that conventional wisdom may be wrong. - Free Online Library