I suspect that this is one of those things that have grown up from earlier times and become entrenched in the system. Maybe it had its' genesis in the fact that each runway heads in a specific direction. In any event, we all make an effort to remain on the centreline during each take-off so aren't we all actually maintaining a track anyway?
That's the way I've always looked at this issue, so an instruction to maintain runway heading means, to me, that ATC expects me to stay on the extended centreline. I figure that this is one of the main reasons why runway data includes the actual magnetic bearing of the runway. I can't see any other interpretation that would be safe in all circumstances, especially when one considers the protection that is afforded in the surveyed take-off area.