This was more along the lines of my thinking too. A more reasonable explanation here is that they this gives them control over UI and security which from MS's perspective may make sense if people are likely to attribute any UI / security issues to MS. It could also be something to do with the analytics they're able to collect.

Also when you have a company the size of MS talking about a small decision like this as if the whole company is onboard and actively engaged in the decision making process is naive. It was most likely a decision made by a few people at MS. Not saying that excuses it, but often individuals in companies have their own misaligned incentives and make certain decisions for their own benefits.