Why has he lowered himself to this.
They were wishing him luck with Wimbledon, so it's nice to return the courtesy. They may not be to everyone's tastes, but I know one of them is a big tennis fan (couldn't say which one as I don't know which one is which) and they are one of the biggest pop groups in the world right now, so it's ridiculous to suggest he's "lowering" himself to be associated with them. I'm sorry, but that's pure snobbery.
They make pop music, and he plays tennis. Neither of them are discovering the cure for cancer, but they both make a lot of people happy, and this is just a bit of fun.