As we have passed from a punishment system that didn't ban teamers to a system that can, the rules have only recently been updated and people may not be aware of the change.
It is therefore necessary to warn them of this change in the system by telling them that teaming is bannable before reporting them (if they continue teamimg)
Until there isn't a propper warning like in eggwars it is necessary to inform them instead of banning them without them understanding why.
To answer your question : Teaming is bannable once the teamers have been informed.