What is to prevent me or anyone from just holding down the return key for an hour

Nothing; but you're only wasting your own time. And there's nothing special about newlines. What's to prevent someone from pasting <HR/> a million times?

and posting 7 million lines of whitespace?

LOL. Node size is limited to 64 kb.

And if anyone posted a node consisting of 64kb of newlines, that node would be reaped within the hour.

So what was your point again? lol