The key word there is "customers". Surely there must be laws against loitering. Someone who hangs out in a restaurant without buying anything is loitering, IMO, whether they're kissing or not. It should be perfectly acceptable for the owners to ask them to leave! If they refuse, the police should step in.
OTOH, if the protestors have to buy food to stay, they're actually supporting the cause they intended to protest.
I had a CFA sandwich yesterday (took two hours to get it) but I could have another one tomorrow while I'm laughing at the protestors...
I think we both know that such laws are applied in a biased way when race, gender, religious and sexual beliefs come into play (heck, just look at what the DOD did with gays participating in parades while in uniform).