"Therefore, pizzerias utilize direct-mail marketing more effectively than do other restaurants."
This is a definitive statement, s/he's saying that pizzerias always utilize direct-mail marketing more effectively than do other restaurants.
99.9% of the time, I'd agree with your intepretation of this. It certainly accords with how we use these terms in everyday language. Not on the LSAT though.
Pizzerias could actually be relatively poor utilizers of direct-mail marketing, but as long as there's at least a token number of restaurants that do it worse than them, then it's technically true that "pizzerias utilize direct-mail marketing more effectively than do other restaurants." It's like saying I play baseball better than other kids, or I'm taller than other people. Neither of those statements imply that I'm a better baseball player or taller than ALL people -- just some. If 20 kids on the baseball team are better than me, but I'm better than the very worst 4, then I can say I play baseball better than other kids.
This is my main beef with the question. If you really get down to the nitty gritty and analyze the wording, none of the answer choices are perfect. This certainly qualifies as over-analysis, but the LSAT trains you to think this way, and I've seen other questions where this type of confusion between some/all was intended as a trap (though none as subtle as this one would be if it actually *were* a trap).