The purpose of the team selector perk for donators is when there is 2v2, 3v3 then you can choose the team that you would like to join.
Like nub14 said The rule "Unbalancing teams on purpose" is stated under Stats Manipulation,
example there is 3v1 3 is the red team and you join them by using the donator perk which is not allowed to and it counts as "Unbalancing teams on purpose".
Staff needs to ask the player first to change teams.
If the player is ignoring staff or does change teams but second after the player sees that staff has left and that specific player decides to unbalance the teams on purpose then we warn them.
But in your situation, you didn't know about it and you got warned, which is wrong we should've asked you first before giving out a warning about it which is what we suppose to do.
I highly suggest checking the rules https://cubedcraft.com/rules/Sincerely, ApexCandy.