It's straightforward for SDM to designate specific products such that no points are given for purchasing them because such designations are internal; i.e. points are given or not on SDM's system, which "knows" exactly what you bought product by product.
For Mastercard's system not to give points (or cashback) on particular items (e.g. stamps), it would need to have the same fully detailed level of information, presumably from SDM. It would also need to be programmed accordingly. I'm not a lawyer and have no idea if sharing such info between companies would be legal, and I also wonder if doing so would raise ethical / privacy questions. Accordingly, it would seem much simpler for MC to give N% on all purchases at SDM.
Keep in mind as well that only a fraction of stamp purchases actually get points back or cashback anyway.

