Russia does. They want to diminish U.S. influence in the region and elsewhere.
History suggests that when US troops are killed, the US does not become less influential - it gets pissed off, sends more troops, and chucks more money at puppet leaders. Most countries are aware of the concept of "blowback", though the US has still to fully grasp it.
Russia has a long tradition of sowing chaos in order to advance their own goals
In this case, there is no need for Russia to do so. The chaotic nature of Afghanistan combined with US incompetence at counterinsurgency, combined with the pentagon, et al blocking attempts at US withdrawal, ensure continued chaos in that area, keeps US forces busy, and keeps the US pissing away money unproductively.
As you said, Putin is intelligent, and wouldn't interrupt the enemy when he's making a mistake.
Unaddressed still: you and other commentators, and the media, denying Afghan agency. They are quite willing and able to kill US troops without anyone else's help. What do imagine is the scenario here? There's some quiet Taliban commander sitting around in his compound somewhere up in the north, enjoying the peaceful afternoon sunshine, after forty years of continuous conflict first against the Soviet Union, then against warlords, then against the US, after his daughter was killed at her wedding in a US airstrike, his first son killed at a US checkpoint, his second son imprisoned and tortured by the Kabul regime, he says, "Well that's enough, I'd like peace, I'm not going to bother negotiating for it though, I'll just sit quietly in my compound instead," and then a GRU officer shows up with a pile of cash and the commander says, "Well in
that case..."
Seriously?
The Taliban may be illiterate goatfuckers, but they're not stupid or cowardly, and they really, really hate the US. They don't need a financial incentive.
The only newsworthy element to me is why didn't the president of the U.S. do anything about it?
He doesn't do anything about
real problems, he's sure as shit not going to do anything about
fictional problems.
In Trump's position, I would keep loudly denouncing this absurd fiction, because that'll keep the Democrats stirred up and focused on bullshit instead of my real inadequacies and failures. Let's hope he's not smart enough for that.