You need to understand how IRC works. Ops in an IRC channel are the owners of the channel (like owners of a house.) They determine what is allowed there. You are merely visiting their channel (or house.) If they do not want you in their channel (house,) then they have every right to ask you to leave or make you leave. (We call them ops, not mods ...and they can ban for any or NO reason on IRC channels, just as your neighbor can ask you to leave his house if he does not like you or does not want you there.)
If you do not like how a channel is run, create your own or go to other channels where you are more welcome.
Your issue has nothing to do with Undernet, rather than it has to do with IRC in general.
Hope this helps.