Hmm...
Well, I actually do see one legitimate reason for Sony to act differently than Microsoft in this regard.
What are promotions designed to do? Promotions are designed to increase the number of people using your product over a certain period of time. They offer incentives for people who would not usually buy/use your product to do so, possibly (hopefully) gaining them as customers in the long run.
According to their website, Xbox Live Multiplayer costs at least $5 a month. If someone on the Xbox decides to play Mass Effect 3 multiplayer on their network, that gives them a reason to spring for Xbox Live. If they don't ever cancel that (which will probably happen in at least a few cases, what I wouldn't give to have all the money I've spent on stuff I don't use any more back) then Microsoft scores an extra $5 a month... forever. Or more, if they spring for the family pack.
On the other hand, PSN usage is free. If Sony runs this promotion and "gains a customer" in a similar manner, here's what they gain: extra drain on their servers, more work to turn this event on and off, possible bug fixes if there's a problem with it. That's it. Taking this opportunity to "promote" their free product does nothing for them but waste money... granted, probably not very MUCH money, but it still gives them... well... no reason to bother?