Google returns more results for “preemption”, but “pre-emption” still returns more than 1 million results (including sources such as the New York Times).
Is there any reason for the hyphen other than readability? It seems like it’s not really related to ‘emption’ as far as I can tell.
What do you mean “other than readability”? Readability’s not chopped liver, you know. 
The main reason is that in English it is very rare for a syllable break to fall between two Es like that. Without the hyphen (or a diaeresis, i.e., preëmption, which is non-standard in modern U.S. English usage) some readers might see the word as “preem-tion,” which would be confusing.
The hyphen clarifies the nature of the word and enhances readability.
Using it or not is a style choice, of course.