Because some people don't use the spacebar for themselves, or use a browser that doesn't support spacebar page-down scrolling, and think shortcuts are smart. Fo

Because some people don't use the spacebar for themselves, or use a browser that doesn't support spacebar page-down scrolling, and think shortcuts are smart. For them it's OK to experiment and remap a key to some feature, even though only them are aware of it, and most other people won't see it and won't use it, or will get bitten as you are. Tumblr is a a particular nest of this kind of websites. That's unfortunate, and funny at the same time. Internet is also a playground, a sand box, a place to learn. Best you can do is contact them, and teach them a polite UX lesson.