They may have controlled publisher pricing. However, it's more likely that they just specified a recommended price point, led by themselves and their 1st party catalogue, and other games had to match due to competition. But Wii games were cheaper to make than PS360 games, so publishers could still make money at the lower price point.How did Nintendo release titles at 49.99 for Wii versus 59.99 on everything else? We've seen pricing vary on other platforms historically based on power.
Point being, the cost of a game in store is broken up to pay various people. You can't reduce that without reducing the money to some of those people. I doubt publishers would be willing to release on XB1 if they have their profits cut. I doubt retailers would care to stock XB1 games if their profits are cut. So MS would have to absorb the cut themselves, destroying the key earner from the console business.