It is, I think, true that a foreign language always poses a limit to the fluency of the user. But just as true is the fact that this can be overcome by communicative learning. And nowadays communicative learning doesnt only mean reading books in that language and then downing a grammer guide, it also corresponds to communicating with a larger mass and then learning through errors.
I, as a non-native user, am not as smooth in English as a native speaker is. And yet, I've gradually grown a steady confidence in this language. I am, of course, still unsure at times about my use of words and sentence-structures (not understanding the idioms and phrases are highly frustrating too), but still I follow my instincts in using english in a better way(since i hate studying grammer and i dont really think it'd ever help me improve my shaky vocabulary) and most of the times they are right.
Thats an interesting question, indeed. Noone's forcing the writers to write in english, and surely they'll be more at ease with their native tongue. But I think it's the desire to share their works with a larger community and since english is massively being used as the global language now so may be they like to "be in the game". Another reason can just be that they're positively smitten by english literature and like to stay in touch with it.
In my case it is the latter that makes me write stuff in English as well as in Bengali. I have loved every bit of what I've seen of English literature till now, it's not much, it's never much, but still it's been enough to make me be in love with it so far.
And so I write and share my small attempts at being part of that wonderful world in my own small way. I'm not going to be a writer, but that doesnt necessarily mean I cant play with what I love.