I know people who are legitimately bisexual like myself. I know people that label themselves as bisexual and I wonder about whether they really are, but it is not my job to label other people or question how they identify themselves. I don't think "pretending" to be bisexual is any better or worse than "pretending" to be straight, although there could be vastly different reasons for these two actions.
As for reasons to label yourself bisexual when it might not be 100% true: Some people feel that taking this in-between step between straight and gay makes coming out easier, like slowly lowering yourself into cold water. Some people might just be bi-curious but they don't know the difference between the two labels. For some people, it is just a phase; there's nothing wrong with that, sexuality is fluid and our preferences can change over time - although it's important to note that it's not like we're choosing to change them, like we woke up one day and said "I think I'll be gay today instead."
You say that you know girls who say they're bisexual but then back off when other girls try to kiss them - this doesn't mean they are just pretending to be bisexual. Being attracted to one sex or another (or both) doesn't mean being ready for physical interaction with them. They might be asexual, aromantic, or have some kind of psychological issues regarding intimacy or their sexual orientation.
That being said, people who truly have no reason to call themselves bisexual other than seeking attention disgust me. They are just giving the rest of the world another excuse to think badly about a group of people who have done nothing wrong, and confusing well-meaning straight folks who are trying to understand our point of view. If they are flirting with the same gender when they are lying about being bisexual, they are leading those people on and that will lead to a lot of hurt feelings and wasted time.
But people who try to tell others that their self-proclaimed orientation is wrong or is just a phase, are just as bad. It's my orientation, it's not hurting you or anyone else, and if I feel like I need to re-label myself a year from now, that's my right! So stay out of it.