I agree, checkboxes would be better. However, implementing as checkboxes would be more difficult, because the nature of html checkboxes. Specifically, a select area can easily be created and extracted into a php array, whereas each checkbox must be created as separate html inputs, and then would have to be re-combined later.
I’ll remember this as a feature request though. If you want to make a patch to add checkboxes, I’ll be happy to test it and add it to the next version!