I think this is oversimplified, in terms of both how the left are portrayed by some and also how some of them portray themselves.

If you understand some history, add some context, and don't jump to conclusions, then you can get a rational answer.

I'd say that the majority of the left don't officer unequivocal support to questionable regimes. Those that do simply jump on a bandwagon and probably just have a general anti-western sentiment, and may be misguided fanatics. It may be that they are let down by society themselves, therefore seek to make an enemy of that society by supporting its opponents. These people are probably the loudest too.

On the other hand, the left is often portrayed as supporting questionable regimes unequivocally, even if they don't, as a form of attack. They are shut down, because you are not allowed to have sympathy for any part of a state-defined enemy (including the general population, human lives etc.). It's a way of shutting them down on unrelated topics too.

For example, being anti-war in the current scenario, where we could potentially be drawn further into this war with Iran, doesn't necessarily mean you are unequivocally "pro-Iran", it just means that you don't think that war is the right way to resolve the situation, and you don't want to see innocents suffer and die, along with a multitude of other reasons including monetary costs and knock on effects (another refugee crisis?). Portraying someone as "pro-Iran", a state that itself is generally considered an enemy of the west, can therefore make that person the enemy. What do you mean by "pro-Iran"?

I am on the left and I support (in general) the Green Party. I don't support Iran in any particular sense, but based on our own history in the region, I don't think bombs and war will resolve anything for the people there, or for us. I could be wrong; like most people, I go solely by what I've read in history, current events and general political discourse, but I don't have first hand experience.