Because Microsoft can make proprietary and patented enhancements to software under the weaker licenses, it can apply its embrace-and-enhance strategy: Microsoft introduces incompatibility into the Microsoft version of the software, and forces the public version of the software out of the market because it won't interoperate with the Windows version. Only a vendor that dominates the market could use such a strategy to maintain its monopoly. The GPL-licensed Linux system is the only one that has been able to make a dent in that monopoly.
Microsoft executives justify their position with the mantra, "We are for strong intellectual-property protection." But only when it's to Microsoft's advantage; otherwise, Microsoft wouldn't be pressuring others to weaken their open-source licensing. Even during the penalty phase of their antitrust prosecution, the company still can't settle for a piece of the software industry pie. Although Microsoft's compromise with the U.S. Department of Justice is crafted to make it seem that a competitor could enter, it still leaves Microsoft owning the whole pie.