Maybe the better way to think about it is Bernie is well within the political mainstream of the Western world. His most radical policies are things that have been in place in every other developed country for decades. Not only do those countries have those things, namely socialized healthcare, but they are immensely popular and even Conservative parties wouldn't dream of taking them away. Are the Tories socialists? Hell, even Bernie's healthcare ideas are far more conservative than what the UK has had in place for 75 years.
You see "socialist" and think Stalin and Mao when he's just a standard Labour party member or Canadian Liberal.