Vikings:
i guess it is because it says nothing about conspiring, and loosing on purpose in this case boosted someone elses ratings, besides he doesn't think it can be proved

nothing about conspiring - it doesn't have to be explicitly mentioned to know it's against the rules. The rules say no losing on purpose in order to boost your opponent's ratings is allowed. That unequvically includes conspiring.

loosing on purpose in this case boosted someone else's ratings - yes, that's what the user agreement prohibits

he doesn't think it can be proved - does it mean that if something can hardly be proved, it should be legallized?