Understanding a foreign language and forming sentences in one are very different. OP understands well enough if he can form sentences that aren't perfectly spelled but still get the point across. His experience with hearing English is different than the way King writes it, which is true for everyone, because King writes really bad modern American dialogue.

I like to give him the benefit of the doubt and chalk it up to regional and generational differences, because he does craft a good story despite the truly awkward spoken words, and when I read The Dark Tower I actually found the dialogue pleasant when it was in the Fantasy worlds, and right back to terrible when it came to our world.