People who do this obviously aren't too interested in keeping the game long-term, which means they would have likely bought it on sale or used, rented it, bought it and then resold it, and so on. Which means not a huge amount of money from those people overall, if any (in case of a used copy, no money at all goes to MS). This user effectively gives 30$ for sure to Microsoft, possibly more if he ends up really enjoying one of the games. If he kept on buying used and such, it's no guarantee they'd effectively have seen more money from him.
While I agree with your statement that MS is making little money of someone that would have likely not gave money to MS at all, I doubt a sporadic $10 would be enough to pay for the hundreds of titles MS is essentially renting off third party every month.