Because it's a term that's come up in ways that it's easy for people to not know the derogatory meaning behind it. Plus, you have some trans people who call themselves the word without any problem doing so, because they don't take it as seriously as others do. So "spending more than an hour on the internet" is no guarantee that you're going to know the full potential meaning behind the word, no.
Plus, characters like Bridget absolutely are traps, so it can probably be hard for some to know the difference between, say, Bridget and Poison, unless they actually have some deeper knowledge on the whole subject of being transgender—which is far, far more people than those who do have said knowledge. There's often way too much assumption on how much general society knows or should be expected to know about what it means to be trans.
Those who know the deeper meaning and rush to post examples like Poison are totally obnoxious people, but I don't think you can just assume everybody falls into that category. Some legitimately believe that "trap" is just a more slang and internet-born term for "trans" (just like how there are tons of people who think "tranny" is the same and don't know that many people take offense to that word).
Anyway.