I can't see how this is legal. Surely it shouldn't be, if the owner is fine with squatters in his unoccupied house, then so be it. Just moving into someones property is pretty bad.
What if you the man linked in the OP was actually trying to sell his house? If squatters moved in and he had to wait 6weeks to kick them out, then he might miss out on selling his house to somebody, who was willing to pay a good price for it.
Or if you're putting a property up for let? I've had to wait a couple of months for a house to be renevated before I could move in. If a group of squaters had moved in, there is no doubt...I'd pick up my keyboard, find a way inside, and kill me some scumbags.