The LEGO games appeal to kids, selling very well (LEGO HP was in the top 10 NPD for two months, I think) and it's also a good way to get a competent game based off of a popular franchise.
I mean, look at the LEGO Harry Potter game. It got pretty good reviews, sold very well, and was, on all accounts, a good game. It didn't rewrite the franchise, but it was another solid entry in the LEGO Based On A License franchise.
Meanwhile, look at the Harry Potter 7 movie game. It's apparently a steaming pile of crap.
So, in conclusion, these games might not be for us, but it's a fantastic way for fans of these franchises to get a good game based on something they adore.