I'm not sure why the promo numbers are considered a cause for concern.
1) Can other publishers do similar promos? Yes, absolutely. If they choose not to or it's not economical for them, okay, but that doesn't preclude the sales from counting. Like Wii Play; Microsoft and Sony could have sold a $10 game with a controller. They didn't. Maybe if they did they would have done as well, maybe not. But they didn't do it. Doesn't make it not count that Nintendo did.
2) Do other promos ($100 GC, for example) count? Yes, absolutely. And they should. And price drops should. A console sold is a console sold, whether it's at a profit, break-even, as a promotion, or at a loss. We don't say that Sony is cheating by selling the PS3 at launch at an enormous loss. We don't say platform holders are cheating by bundling games with their consoles, or offering holiday promos, or per-game special edition consoles. We don't say platform holders are cheating by cutting their price. All's fair.
If Microsoft thinks it's economically viable to run a promo that sells 200k 360s (assuming 100% of the month-to-month growth is account for by the promo), then that's their prerogative.
I guess my point is that if you're asking "Why is the 360 selling more?", the promo is a good answer. If you're saying "Because the promo was responsible for the growth, the growth doesn't count or isn't impressive or doesn't mean anything", that makes no sense to me.