I agree, you should not be responsible for paying for their drinking AND enabling their laziness (in not wanting to take the time to fairly split the check.)
Some friends of mine had problems with another couple they would go out to eat with. Mr. Friend of Friend was very "generous" and would order appetizers, drinks, wine, and dessert "for the table." At the end of dinner, he would tell the waiter to split the check down the middle, leaving my friends to pay for half of starters, drinks, dessert that they hadn't planned on ordering and wouldn't have ordered if Mr. FOF hadn't done it. After he pulls this on them twice, they stopped him after he ordered an appetizer (waiting until after the waitress left the table) and said that they were not going to be splitting the check with him and that they were not planning on ordering wine, cocktails or dessert as they do not normally do that when they go out. He got mad, but he also didn't order drinks or dessert. The invitations to dinner stopped shortly after, which to me showed that what he was looking for was someone to supplement his dinner costs.
You just have to stand firm. Or stop going out with them if it really bothers you.