Earlier quoted context omitted.
The backspace thing has gotten attention more than ever on sites like Reddit. I have routinely seen posts complaining about it and the spacebar key doing behaviors that users never ever intended. For example, pressing space and then accidentally scrolling down which wasn't their intention at all, but is the behavior of space. I think some of those complaints reaching the front page of reddit may have helped spur this…
Spacebar scroll is standard on many Unix utilities, starting with pagers such as less and more. Given that paging down a screen of text is very likely to be one of the most common things you'll do, and that it's a well-established motif, as well as the fact it doesn't destroy user-state (as back-nav on backspace does), it's quite defensible. I've never actually seen someone complain about it until now.
Frankly, I would be perfectly happy if space to page went away entirely. I exclusively use two finger dragging on laptops and scroll wheels on desktops to browse websites. I never use space to move down a whole page at a time, and that's a jarring action anyway because it inevitably causes me to lose the context of where I was reading in the document in a way that smooth scrolling accomplished through other means does not.