The compensation of servers is intrinsically impacted by minimum wage laws. Servers are a special category and their employers are permitted (and expected) to pay them less than $3/hr, on the presumption that the difference between their paid hourly rate and the statutory mininimum wage (for everybody else) will be made up by the tips they earn.
Thus, a tip of approximately 14% is a part of their compensation that your are ethically obliged to pay, unless the SERVER has been conspicuously deficient in providing the required services.
Mandatory tips for groups of a certain number (or larger) are appropriate because large groups tend to occupy table space for twice as long as typical patrons, thus limiting the opportunity for servers to earn more in tips from replacement customers.
If you go to a restaurant not expecting to pay the additional 1/7 of your tab to the server then you are uninformed and possibly worse (selfish, unkind, ungracious, spiteful, and cheap). Stay home.
Tangentially, I (being a cheapskate of great renown) resent the "Tip Jars" that one sees in walk-up eateries like Starbuck's, Panera, Brueger's and so on. The "baristas" (and whatever other titles they have) are NOT servers, and are thus entitled to the standard statutory minimum wage. Most make more than that, judging by their appearance and demeanor. Tipping them should only be for extraordinary service, but makes no sense in any event, because the contents of the Tip Jars is shared socialistically. Pity.
Tips, when appropriate, should also consider unusual factors unrelated to amount of the bill. If you are dining with friends and will be occupying the table for a much longer time than usual, an extra tip is appropriate. If you have small children with you, or anyone who requires special attention, or will make an extraordinary mess that will have to be cleaned after you leave. Leave an extra tip. If you are like my brother-in-law and chronically flirt with the waitresses (and they don't pour water in your crotch), leave an extra tip.
You get the idea.
It ain't optional (you cheap bastard).