I know this is an age old argument for Malte...and I can always see two sides of a story. I think nowadays people think of fanfics as a chance to make cameo appearances of their favourite characters in a situation they is most appealing to them. It's especially gritty with LBT because of the lack of technology and human understanding that, I think, a lot of fanfic writers find an insurmountable barrier to try and conquer. So, in light of that, they decide to destroy the barrier by removing it altogether. I'm not saying it's a pathetic way out or cheating, but it just makes it that little bit more easier to apply new rules to patch up the old ones they've decided not to abide by. I'm no saint in the matter as though who have read my LBT fanfic all well know of what's included but, I reckon people feel their imagination is threatened by the idea of NOT being allow to include things like "alien crafts crash-landing in the Great Valley" or"humans time travelling back to the dinosaur time" for whatever reason...
It's all down to the author and yet, it is not the reader's obligation to decide what is included. They may make suggestions but it's down to the writer to have the final say. That and the reader is again under no obligation to read the fanfic if he/she feels it doesn't apply/interest them.