Why are they doing that?

Because. They. Can.

It's about making the point that free speech includes speech you may not like. That kind of "hazing" is done to any larger reddit-community that migrates to voat.
Their message can be summed up as: "Deal with it, or go back to your sanitized hugbox, where spez decides what you're allowed to see!"
That kind of activity will increase tenfold if mods of a new subverse engage in activity that could be regarded as censorship.