This is a good point. However, the fact that the US embassy predicted a few weeks back that there would be a attack on... perhaps a theater (though in the two days following).

On the other hand, if you're planning a terrorist attack yourself, then you probably wouldn't say that. It's not as if they care enough about US citizens to sacrifice a few in order to gain more plausible deniability. On the yet other hand, they may have been aware that Ukraine was planning something, while knowing that Ukraine killing even more US citizens would harm support for Ukraine in the US.

Now, it is also possible that they don't care much about being blamed, and this was a way to show the incompetence of the Russian government. Even the US embassy knew that a terrorist attack was imminent, and yet the incompetents were still unable to avoid a terrorist attack. This does make the Russian government look quite bad.

Who knows? Maybe in decades we'll figure it out. Or maybe we'll have long forgotten about it, just like people forgot about the Las Vegas shooting.