yes, they became an awkward part of the daily life in cities since the bataclan attack. see Vigipirate. But they are not particularly guarding synagogues. You can see them in front of churches or in train stations or touristic and popular places too.
if you needed to add "leftist" to your name, i suppose you're familiar with The Society of the Spectacle. They are not there to protect you, they're there to make you feel protected (worse, to make you feel like you need constant protection!)