Portal may be free on the Mac for a limited time, but nothing's ever really free. It's just being subsidised. Sponsored by a benevolent artificial intelligence, who is only too glad to motivate you for Portal 2.
The term "marketing engineers" sounds made-up, but in Valve's case, you can never be too sure. Ideas like this - and those for Team Fortress 2's never-ending stream of updates - don't come from the minds of laymen.