Who do you think are behind all those DOS attacks?

I suppose that's possible.

I had my own theory (though it may seem bad). The firm that set up the blockdos attack thingie must need customers. Therefore if they themselves attack the sites they then have a customer

No eh?

All i know is that the attackers blackmailed the owners of the PTC sites to get some money off them and then they would stop attacks. But they were stupid enough to have accounts in those sites attacked so their IP's got blacklisted. That has lead to the ptc sites using blockdos and that means big money to the blockdos people so as Mysdianait has said, the blackdos would also play a part to increase their revenue.