Come to think of it, would Visa/MC be doing the conversion? I believe that's the issuing bank's job. But they offer the rewards, not Visa/MC, so it doesn't change the original answer much.
The rewards are funded by the banks via interest income. 3% cashback cannot be covered by interchange fees, for instance.
VISA/MC do offer the spot rate. But the idea of a spot rate is a little iffy anyway, as it assumes that there’s a counterparty willing to sell/buy at that price, and at the volume you want to transact at. When you’re operating at MC/VISA scale, that isn’t a given, and neither of them are interested in taking on FX risk. So their spot prices will be close, but likely not identical to spot prices you might find at other FX providers. Over the long term though, I would expect any delta to balance out.
A lot of banks do charge a percentage fee on top of Visa/MC's exchange rate, but my bank does not.