Suspect you might just be a tad over-negative about this.
First, Microsoft doesn't own any factories so the business is going to a partner (probably Pegatron).
Second, Microsoft partners can sign up to sell and support the Surface Pro range. Dell and HP have already done so. Shipping thousands of Surface Pros into businesses, imaging them, and supporting them on-site and remotely is the part of the PC business with margins, which are rather thin on Windows hardware.
Also, Microsoft isn't competing in the mainstream Windows market, where device prices are typically $200 to $500. In fact, it's mainly competing against Apple, which is arguably a good thing for the rest of the Windows ecosystem....
The Nexus devices were often also available straight from their vendor at the time in the same form or a slightly different model.
And most importantly the Nexuses were always sourced from on of their partners, here MSFT is directly competing against their partners.