> he is being arrested because it is illegal in Russia to be against the war

do you have any source to support this?

I went thru most "Criminal Code of Russia" searching for anything related to war, and could not find anything in terms of even temporary jail time for speaking out against war. Many people do, in Russia, including politicians of Putin opposition and they are nor being jailed. Looks like you can get jailed for participating in a protest against war or basically against anything in Russia... but nobody is going to bother going after you because you have posted a mean tweet. And if you search Twitter there are thousands of anti-war anti-Putin tweets from Russian accounts posted for years and these accounts keep being acting and keep posting (from jail I assume???).