I imagine the rationale is that Russia wants to have Africa as an ally, and booting out western interests shrinks the power the west has.

Niger’s current government demanded that the US troops GTFO and these Russian troops are a “not Wagner” replacement for Wagner contingent that was in the country already. Officially there to strengthen relations and train Nigerian army.

As for barging into an American base while their troops are still there, that has to be a démarche of sorts, much like that message at the trophy display in Moscow.

I skimmed through some American statements about the whole thing and this might be Kremlin preventing Americans from stalling, because they said they’re pulling out, but want to keep presence for as long as possible, maybe waiting for an opportunity, considering situation in Niger isn’t exactly stable yet. Nothing like a threat of a major diplomatic incident to make people hurry, I guess.

P.S. The message I mentioned and translation for non-Russian speaking comrades: “Attention! Employees of the United States, the UK, Germany, France and Poland embassies are granted access to inspect trophy NATO equipment OUT OF TURN.”