They can "clarify" as much as they want until they're blue in the face.

It's what's in the contract that matters, and every residential ISP contract I've ever seen has acceptable use limits, tells you not to run a server, etc.

The ultimate catch-all is language to the effect of "activity that negatively impacts other network users" or some such.

300GB is arguably acceptable use. If they don't like people going over a threshold then they should write down a clear number (even) in fine print, *acceptable limit should be under 300GB or under 200GB or whatever limit works for them. Just because the marketing wants to sound one way they should not confuse a customer who wants to use the service they purchased

It's pretty clear from the first paragraph of their acceptable use policy...

1. Use. The Service is designed for personal and family use (residential use only) within a single household. Subscriber agrees that only Subscriber and Subscriber''s authorized guests in the same household will use the Service. Subscriber is responsible for any misuse of the Service that occurs through Subscriber''s account, whether by a member of Subscriber''s household or an authorized or unauthorized third-party. Subscriber will not use, or enable others to use, the Service to operate any type of business or commercial enterprise, including, but not limited to, IP address translation or similar facilities intended to provide additional access. Subscriber will not resell or redistribute, or enable others to resell or redistribute, access to the Service in any manner, including, but not limited to, through the use of wireless technology. Spectrum reserves the right at its sole discretion to immediately suspend, terminate, or restrict use of the Service without notice if such use violates the AUP or the Terms of Service, is objectionable or unlawful, or interferes with Spectrum''s systems, or Internet Network, the Internet, or others'' use of the Service.

It's a 35mbit up connection. His usage reprents over 95% utilization 24/7.

None of that says you can’t do a 300 gb backup every day. He’s not sharing with his neighbours or running a warez server.

Clear how? Point out the specific clause you say he's violating.

Yes, this is not clear because that would ruin their marketing mojo. Either let this guy use the service or fire the merketing head or accept the cost of poor marketing and let people use the service uncapped. Bad press is worse than a fineprint with the actual cap. Reasonable service is not describing anything. Reasonable is 1GB for some, 500GB for some, 5TB for some. Bad press will make it clear this was deceptive advertising and there is no real unlimited option.

Clearly the customer picked a service for their need in order to use backup: unlimited bandwidth checked that box.