"I know I'm a tiny minority when you see every major cruise line in the world featuring overnight stays in St. Petersburg."

I absolutely share your opinion re the current regime in Russia but I would pay a pretty penny to see -- not  Petrograd -- but the Hermitage Museum there. I presume there is an expectation that more of my dollars would be sought -- hoping I might return to spend more dollars elsewhere in the country. But, no, I prefer to overnight on board the cruise ship I arrived on and will depart on. If I chose to return to Russia, it, again, would be to visit the Hermitage -- far too much to see/study than could be accomplished in a single cruise visit. 

That museum is a marvel in this world and I choose not to ignore it. Have you actually avoided travelling to any country headed by a non-democratic regime?