It is a free kick, you can run across and get in the way a bit, not make yourself easy to pass but you can't run across the pitch and purposefully get in someones way. If he'd gone over and gotten close, or been there to start with and not move, no freekick. It's a weird one and looked soft, but most players simply wouldn't be stupid enough to actually make the block, just get close enough to put them off a bit.
The even more stupid thing is the guy wasn't even motoring to get back, he wasn't killing himself to catch up, it was a token effort, utterly stupid to block him as he was no threat to whoever was on the ball anyway.