These people actually think they are freedom fighters and want to restore "good" empire.
Why would anyone register a domain with such extension is beyond me unless they are completely brainwashed by the regime.
Here is example of a typical "news" site from that list: sevastopol.su
You can use Google Translate to understand the skewed picture that is being tried to shown to the local people.
Because it's fun? Most sites where probably registered before the war and aren't necessary serious. You can have a good time with a soviet styled website or even community, all with matching url and flags and stuff.
Plenty of anti-Russian Ukrainians use .su