ICE isn't the border force. All border security operations, stopping people from crossing and customs inspections and such, are done by a different agency called CBP.
Most people trying to boycott ICE don't think undocumented migration should be legal. They just don't think ICE is a good agency. (Again, ICE is not the only agency responsible for undocumented migration, so undocumented migration isn't automatically legal if they can't do their job.)