That isn't all the Chechens specialize in. IEDs, anti-armored vehicle attacks, and the newer, deadlier FPDs (Foreign Projectile Devices - "anti-every vehicle known to man" explosive devices made with a conical-shaped copper core designed to penetrate the thickest armored vehicles we have and will penetrate a Bradley and Abrams like a hot knife thru butter), and general small unit tactics (CQB, breaching, capture/kill missions, etc) is what the Chechens are very good at doing.

Better to put them to work for you, than risk them being put to against you. There are a bunch of other hard line Islamist aligned Chechen groups that are still in Afghanistan, Pakistan, and the greater Hindu-Kush area running around with AQ and IS, though.

Edit: And if the Chechens get bored, they will fall back to default programing and start messing around with attacking Russian military targets in and around Chechnya, Georgia, and Russia proper, so I don't see where Putin had any real choice but to use them after we pulled out of Aghanistan.