I would think that's a bug in the online chess implementation isn't it?
Why should an opponents pre-move prevent a player fairly claiming a draw?
Yes.
It shouldn't.
Unfortunately, that's just the way it is, at the moment. I believe the site is getting a rebuild, later in the year.
The problem is that your opponent may move before you have a chance to claim, especially with pre moves. Or on online chess with conditional moves.
I would think that's a bug in the online chess implementation isn't it? A player should be able to press the draw button and claim a draw any time before his own next move, not his opponents next move which may be a pre-move. Why should an opponents pre-move prevent a player fairly claiming a draw?