1) Windows OEM licenses cost nearly nothing to vendors. Like 10$ or whatever.
2) You know all that stupid crap that comes preinstalled on any Windows PC? The jerks who made that crap actually paid the vendor to get it preinstalled.
Add points 1 and 2 together and you'll find that it actually costs vendors a *negative* amount of money for preinstalling Windows compared to Linux or no OS at all.