Honestly no idea. I'd imagine that's just in case they change their mind? The email itself was very poorly thought through (and is honestly quite arrogant).
It seems common for big companies to put things in their legal agreements that are stupid and unfair and are never intended to actually be used. I don't know why.
Because there's usually no downside? The same reason people put those goofy "this message is for the intended recipient only..." notes in their sig file. It almost certainly has no legal force ever but it doesn't hurt.
