There is a difference between the deletion logic for teams being used in template versus schedule tasks:
If a team has been assigned to a task(s) in a template, then it cannot be deleted. To delete the team, first remove it from all tasks to which it is assigned.
If teams are used in schedule tasks, then the deletion is allowed but it is a logical deletion. For example, the schedule task can still see the team even though no new tasks can use the team.