1. The OS isn't worth this, in which case it's either extortion (if their partners feel they have no choice) or cluelessness (if their partners end up walking away).
2. The OS is worth this, in which case they've over-engineered: they made their particular OS do way more than it's supposed to and are trying to recoup the cost of developing so damned much.
Fundamentally, an OS needs to boot a device and provide resource management. In order to be compelling to developers it needs to include good libraries that make common tasks easy and tricky tasks efficient. ARM tablets don't even need Windows' backward-compatibility, so there should be a heck of a lot they can just drop out of it! It is hard to imagine this OS being a beast; apparently it is.
Technical points aside however, Microsoft is also the underdog, and a rich one. If anybody should be eating the true costs of an OS and charging partners $6 a head to boost adoption (whether or not it's worth $85), it's Microsoft.