You do make a good point about some scandalous portrayals of historical and fictional characters. However, let's examine them one by one.
You mention the 30th Corps portrayal in A Bridge Too Far, and the insinuation by Robert Redford that the British didn't rescue the 1st Airborne Division because they wanted to drink tea. Firstly, dialogue in that scene revealed that the British tank crews were under orders not to advance without their infantry, which hinted that there was a military reason not to move forward - gently, perhaps, but there. Certainly the difficulty of moving forward in the terrain in the area was amply illustrated throughout the film. But the entire film had a pronounced "military stupidity" slant that a discerning eye would notice. Again, I think an intelligent viewer who actually cared what a 30th Corps was would walk away understanding that. I hope they would go read a book and find out for themselves what 30th Corps really did, and why, and in the process find out some far more interesting things than what some European director thought of how fast they moved.
The Devil's Brigade was in another league; the character of Major Crown was obviously fictional, as was Major Bricker and just about everyone in the film. The uniforms were ludicrous. The Force itself was depicted as pitifully small, the equipment not even close (not a single Johnson LMG?) and the history barely recognizable (no trip to Alaska, for starters). I didn't even know there were two officers with the same name until I read your comments on this forum yesterday. Anyone thinking that they might glean historical information from the film I have no pity for.
Brad Pitt as a member of the FSSF - again, he'll be an obviously fictional character. The only way he can "tarnish" the reputation of an actual unit is if anyone is foolish enough to mistake entertainment for reality. If people walk away with a flawed understanding of the FSSF because of the movie - really, who cares? At best, they live in ignorance the rest of their lives. Maybe some will seek out books on the Force and seek to educate themselves with a new found interest. I can't see the harm, up until the point - as Ed suggests - that the film starts to be shown in classrooms and presented as "truth". It's not that kind of movie, fortunately.
I will agree with you, however, that what has been lost is an opportunity to "do it right". But until such point as someone wants to make such a film, there's no point complaining about those who seek to have some fun with the material available to them.
I can't imagine anyone making a movie about the sinking of Titanic that would have been anything as commercially successful without the love story angle. And it may pain both of us to admit it, but I doubt anyone will make a cinematic tribute to the FSSF that anyone will flock to that will ever match in terms of commercial success what Tarantino is about to make with his Brad Pitt splatterfest.