You CAN eat rabbits that are killed in the summer, but it's generally not a great idea. If you cook it at the right temp it will kill all the parasites and worms, and you wont get sick from it, but I'm not sure cooked parasites and worms taste very good.. Never tried them. lol
The reason most people dont eat rabbits or squirrels in the summer is because parasites breed like crazy during the warm months, and any animal killed in the summer is going to be infested with fleas, ticks, lice..etc... Same goes for parasites on the inside. The warm weather makes the animals great breeding grounds.
Also, during the summer, there's a pretty good chance that if you shoot a female rabbit, she's going to be pregnant.
My grandfather used to say not to kill rabbits or squirrels in any month that didn't have an R in it..