Yes they are against TOS. Usually only mom and pop shops have a policy. No one enforces it, though. The reason is there's usually a base charge..like $.30 + a percentage fee like 3%.

So if you buy a $1 soda, that mom and pop store just lost 30% of their revenue, plus 3%, and maybe sold that soda at a lost to you.

For places like McDonalds, they negotiate much better credit card processing fees and I don't care - plus they make a lot of money anyways.