They volunteered because the monthly salary is more than most Russians earn in a year, especially in Southern/Eastern regions with no opportunities. Most sign up for the money, nothing more or less.
We had a similar situation in the UK following protests/riots in summer 2011. People caught up at the scene got attacked by police and layer sentenced, while someone who took a packet of chewing gum from an open shop front after the crows had dispersed was jailed for over a year. The Director of Public Prosecutions who oversaw this is now the leader of our "left wing" party...
Ultimately, state apparatuses are going to come down extremely hard on mass violent action because they represent a material threat to the state. Obviously these riots aren't going to topple the 5th Republic, but if the state was seen to be unable to control them, or was forced to compromise with them through leniency, then it shows a weakness which makes this more likely in the future.
As it says in the article, he's been designated a PEP because he allegedly took money from the Russian government. He denies this but hasn't sued the person making the allegation. Even in his rant on GB news he lists Ukraine first as a potential state known for bribery. He excuses Russian actions at every turn, it's clear he's getting something from them.
Not saying it's valid or that we should excuse it etc. Simply pointing out that the fact that the majority of Russian soldiers currently in Ukraine are volunteers doesn't necessarily mean they're all hyper-nationalists who love the war