This is a perfectly logical thing to do, though we don’t do it, to be clear. However, as the rules don’t prohibit it, two guilds could benefit greatly from such a pact, as they would both score near or at 100% wins and thus both be elevated, and more likely to gain greater weekly rewards. From an objective game-theory point of view, it’s the most logical thing to do.
Stopping it would be pretty easy - they’d have to just incorporate loss stats into the overall guild score, so that “throwing the game” and getting 100% losses due to crappy defenses would cost a guild so much in points that it would outweigh the gain from making that kind of deal.