I read somewhere that the localization-studio (of ... I forget what show) were forcing the dub-actors to imitate terrible line-readings instead of acting. This doesn't explain the studio's reasoning, though ...
If I had to guess, though, I'd blame the otakus who insist on leaving in "-chan" and "-sempai" unlocalized because they want an "'authentic' 'Japanese' experience."
In my knowledge there are two reasons why dubs tend to be awful:
1) Anime publishers won't pay SAG union rates to voice actors, thus they can employ only newbies who haven't joined the union or those who are willing to secretly do some work on the side. The latter usually get credited under creative pseudonyms like "Dick Smallberries Jr."
2) Everything is recorded in a single take as quickly and cheaply as possible. In the case of the Madoka video, the quality's awful enough I have to wonder if they had the actresses record those lines themselves using computer microphones, rather than getting them together in a sound booth.
Of course, those both stem from the same root cause- anime publishers are really cheap. If you look at voice acting gigs with a budget, like video games, Disney's Ghibli releases or any 3D kids' film, most at least aren't embarrassingly bad. Like Matsuri suggested, maybe anime dubs are just being done to appease the devoted fans at this point. (On the flip side, I'm sure there are plenty of people who won't watch dubs no matter how good they are, so really... why bother?)
I don't know why one would deliberately tell voice actors not to act, but I do know the consideration of matching audio to lip movements can result in some pretty stilted English dialogue. I have doubts the typical dub bothers with even that much in the way of direction, though.
