It's just the "dumbing down" of society...
It sort of like grades at school... In the past, a C was considered average work, a B was above average, and an A meant exceptional work. You could see a bell curve form around that criteria where most of the students were getting C's, some getting B's, and only a few getting A's. But as more students were wanting to get into colleges ("because you need to go to college to get a good job, and I deserve to get a good job just because I exist, therefore I deserve to go to college"

) B's and A's became more of the norm. Fewer people are getting C's and it is a true rarity for a person to get an F.
It
should be that a tip is given for good service as a bonus, not a standard. However, society has been skewed to the point where 1) waitstaff are getting taxed on their "expected" tips regardless of whether they receive them, 2) the minimum wage for waitstaff is less than it is for other jobs, and 3) it has become expected to tip regardless of how good the service was.
To put another way, a manufacturing company, say ABC Tires, gives bonuses to employees that produce more than their quota. We as a society shouldn't start "dumbing down" expectations just "to be fair" to the tire worker that doesn't produce as many tires. We shouldn't be expected to pay a bonus for someone making just enough tires to meet quota, or even, heaven forbid, if they don't meet quota at all. Moreover, the government shouldn't look at the bonuses that tire manufacturers give to their employees and say "since they're making bonus money, their minimum wage could be lessened to account for added bonuses" and tax a certain level of their "expected" bonuses.
I think once government started making rules about waitstaff, it became normalize in society and all bets were off. It is now expected
by the government to tip them... If the government didn't make any special cases in the minimum wages for waitstaff and tax based on expected tips, then I'd feel better about not tipping for average or below average service. True, the food prices would be higher to accommodate the higher wages, but I'd rather have that than the system we've currently got.
But since we are living with these goofy rules, I tend to tip 10% for the bare minimum amount of service, usually 15% if they've come by after serving the food to check on how the meal was, and more if the service was truly exceptional. I have also not tipped if the waiter was rude, goofed up my order, etc. Some cases I've tipped a manager if, say the meat was cooked properly and they give me the meal for free (or discounted something else). I also tipped a busboy once because I saw how after clearing the table, when putting down new settings he'd make napkin origami (but the other sections didn't have anything special like that).