It's to prevent level 50's from joining a group that doesn't want level 50's in their flashpoint! Content needs to be appropriate for the right levels. I would be pissed off if i always got some 50 in my group for Black Talon.
You're going to an extreme. There are plenty of 50s that haven't done mid 40s FPs due to lack of population. Denying them access is silly via the Group Finder is silly. Denying a lvl 15 from doing BT is silly. Denying anyone from doing any lower level content due to a lower level range is plain silly. You want MORE grouping and socializing, not less.
While it's true having a 50 in a normal BT run will trivialize it, it doesn't mean that the 10s-12s in that group will be unhappy. In fact, I'm sure many will appreciate a fast, safe run with one less person rolling Need on gear. And it means that 50 (if farming social points) might be running BT all day, which in the long run, ultimately helps people starting new characters.
The fringe case of a people not wanting a higher level to "triivialize" their content is probably a much smaller group than those folks that would be happy to have a higher level if it simply meant that the group can actually happen.
Just my 2 cents.