It would be nice to be able to retract our mistakes whenever we wanted too, but Jeff has voice his opinion against implementing a way to retract close votes (and that question has 250+ upvotes). I think the whole idea of preventing retracts is so that people will think about what they're doing before they actually do it. In the case of close votes, you can always vote to reopen, but in the case of moderator flags you'll use 10 flag weight for the invalid flag.
Misreading something can't really be considered a good reason not to lose flag weight, because that's the exact type of thing they'd be trying to prevent by disallowing retracts. All I can say is read more carefully next time, and it's not like losing 10 flag weight for a bad flag is the end of the world. I'd be more worried about losing 2 reputation from a downvote than 10 flag weight. Sure, retracting a flag is a good way to prevent moderators from having to look at it, but I think they're more worried about the bigger picture.