My first thought when reading the title was similar: "well, what about pushing it just because it's the right thing to do?" (admittedly, the definition of "right" won't be universal…). But you have to remember that it's easier to use economic arguments with politicians.
If Microsoft were to say "we support gay marriage just because", politicians on the other side could just shrug and not take a risky stand for their re-election. By saying that they are having issues hiring top talent Microsoft is hinting that maybe, possibly not allowing gay marriage would mean that they'll be less competitive, thus less money for the state, less jobs, less economic activity, etc. which politicians respond to. Plus it gives politicians arguments to justify themselves: they don't have to then say "well, it's the right thing" and upset some people, but instead "I'm doing it for the economy".
If Microsoft were to say "we support gay marriage just because", politicians on the other side could just shrug and not take a risky stand for their re-election. By saying that they are having issues hiring top talent Microsoft is hinting that maybe, possibly not allowing gay marriage would mean that they'll be less competitive, thus less money for the state, less jobs, less economic activity, etc. which politicians respond to. Plus it gives politicians arguments to justify themselves: they don't have to then say "well, it's the right thing" and upset some people, but instead "I'm doing it for the economy".