Apparently if you set the sprinkler chance to 0, it constantly fertilizes.

Um? :)

Hmm... Hmmmmmm... Shouldn't it be impossible to divide by 0?

Well it depends whether or not you like watching the world burn

Just tell me that the sprinkler didn't just fertilise but also burn the world down??

This is intended operation.
The rate is "How many ticks until I fertilize", or "How often will I fail to fertilize".
From these perspectives, this is intended :)

Also I don't think the world will burn... unfortunately.

Reopen if I'm wrong of course. :)