First, even where property rights are accepted, a well considered definition generally has a caveat is made that property can't be taken "arbitrarily" or "unlawfully" or through a "legally established public necessity" or so on. In practice, every society which has had a legal concept of private property has also simultaneously had the concept that the government could certainly seize said property under certain conditions, and this has generally been treated as legally legitimate and not illegal robbery so long as it serves some common good. If you don't have such caveats, you run into all manner of issues justifying things like:

>Eminent Domain

>Taxation

>Court ordered seizures

>Civil forfeiture

You can also just just straight up deny the right to private property exists altogether. And not even necessarily from the obvious left wing perspective, you can just say the entire realm belongs to the king, so the king taking things to redistribute as they see fit is not theft, since everything was the King's to begin with.

Beyond those arguments, the probably most crucial argument here is that the poor's private property was stolen to begin with through government corruption and unindemnified torts to end up with this status quo, and how is restoring private property to its rightful owner theft? It is the very essence of protecting the right to private property! Particularly when people are calling for this private property to be restored through legal reform or through government overthrow (legal reform through extraordinary means). Not through simple anarchistic robbing of people who exist nearby you.

As for why people don't simply roll over and say they're pro-robbery, if you robbed somebody, you are a criminal and what you possess is not your property and subject to restoration to its rightful owner. People want to go beyond robbery, they want it to be legally established that people have absolutely no legitimate right to what they currently possess. If you start out on this path by calling what you're doing "robbery" you're already fighting an uphill battle before you even get started.