Well - I wouldn't say there aren't sometimes bad vampires who might. Outside of the blood of the pureblood, or a lover, human blood is supposed to be more desirable than that of an ordinary vampire. We have the tablets though.
[She makes a face. They aren't all that good. And she won't mention that she's heard that the blood of children among humans is especially desirable.]
In the end, it's fine to drink the blood of another vampire. The hunter's association as well as those within thee noble class and above would stop a vampire if he or she was indiscriminately attacking humans.