Not sure why you're having a problem comprehending this. If you ban rock concerts, you're going to eliminate the problem of kids getting stoned at rock concerts. No? If you suspend the speed limit laws, police will write fewer tickets for speeding. Right? This is nothing more than the law of statistical averages. Fewer avenues of corruption means less corruption.
And yet , putting a toll to ensure everyone pays for a service works. It even works if you make the larger trucks pay more.
And it works even beter if an automated system collects the money from an account linked to a tag.
So we have :
a ) A regulation which is straightforward : you must pay a certain amount to use this service
b ) A barrier ( it is really hard to pass the toll without paying...unless...)
c) A control mechanism ( the collector at the toll or a camera plus the tag reader) .
Now , regarding my previous post in which it is not the rules, but the simplicity and the rule of law ( the control mechanisms), I will digress into an anecdote.
While I was on a trip in Colombia some twenty years ago , I noticed the toll barrier was a rather sturdy heavy metalic which had to be manually operated by a soldier and not the usual light piece which could be opened automatically by a machine . I then asked the taxi driver why did they put so much resources into the toll system.
"You'll see", he explained," We used to have light barriers, but then sometimes large trucks would simply accelerate and shatter the barrier, this would happen because they either had no money to pay the toll or because they were carying drugs and they would not risk to a random inspection. So now the army is in charge of the toll barrier."
Now, given the actual situation at that time, and following your line of thought, one could argue that forcing the large eighteen wheelers into paying is useless , because they would find the way to pass without paying ( specially if they had certain amount of illegal goods) , so only cars should pay a toll or no toll fees should be collected at all.
That would be a solution , indeed. But is it the only solution ? or the best solution to the problem ?
Final note: I was told that in spite of having the army waching the toll , from time to time a truck would still try to pass without paying.