Just my 2 cents...
Paid apps have a 24 hr trial period, so to speak, as they can be purchased and uninstalled within that time period. Actually they do not process the charge so its technically not a refund. I'm sure you are aware of this but maybe not.
So to me the trial seems unnecessary. I think it is a nice gesture on your part, but obviously someone does not feel the same. Being a theme dev myself, I know a bad rating, especially when you have very few, can kill your sales. If it were me, I would remove the trial versions.