[popdog54]

With Sprint tethering is clearly a violation of the TOS according to the Subscriber Agreement (highlighted portion by me):

Specific Terms & Restrictions On Using Data Services
In addition to the rules for using all of our other Services, unless we identify the Service or Device you have selected as specifically intended for that purpose (for example, wireless routers, Data Link, etc.), you can’t use our data Services: (1) with server devices or host computer applications, or other systems that drive continuous heavy traffic or data sessions; and (2) as a substitute or backup for private lines or frame relay connections. We reserve the right to limit or suspend any heavy, continuous data usage that adversely impacts our network performance or hinders access to our network. If your Services include unlimited web or data access, you also can’t use your Device as a modem for computers or other equipment, unless we identify the Service or Device you have selected as specifically intended for that purpose (for example, with "phone as modem" plans, Sprint Mobile Broadband card plans, wireless router plans, etc.).

From a legal standpoint I think the only thing a user is guilty of is breach of contract which would result in paying extra service fees and/or termination of service. From a moral standpoint I guess it's up to the individual user.