The thinking is that MS' needs come first, and yours are way, way down the list, unless you're a volume license holder. So magnitude or logic of the gain doesn't factor much unless a policy hits the news with a stickiness of several months.

and it's a pool not paying them for services so they probably don't care about losing them.

So then why bother targeting them in the first place?

If the numbers go up, somebody looks good. Maybe even gets a promotion.

They don't need a lot of conversions. If 2% of users take them up on the ad, that's an increase in revenue and profit. If it pisses off 40% of users, that's not a problem, because only 0.001% of users are going to get so angry that they'll stop using Windows (and those users probably paid for a license anyway as part of the computer purchase, so it won't affect MS's revenues).