Well... why not? Integers would result in a lot of rounding - the guy who earns 0.4 Bells per post would be depressed and the guy who earns 0.5 would be ecstatic.
Fun Fact: You can't send 0 Bells to people, but you can, for example, send 0.000001 Bells. Have fun with that knowledge.
i assume because it has to do with word count, since more words in a post means you earn more bells. so one word or something is probably worth less than 1 bell, which gets added up. if it were an even number at 1 bell a word you would get a lot more bells per post, and it'd be way too much earnings lol.