In US universities, are the sport coaches typically considered tenured professors?
No, athletic coaches are generally not considered faculty and do not have tenure.  I've never heard of any place where they are.  

The conditions of their employment would be based on whatever contract they negotiate with the university.  Typically the university would have the right to fire them for any reason, including poor performance of the team, but the contract might call for a severance payment in some cases.

(There could be small schools where someone is a professor and also an athletic coach.  In such cases I'd expect they would have tenure protections in their capacity as professor, but could be removed from the position of athletic coach at the university's discretion.)