Can ISP force installion of unwanted/needed software?

I have a relative who lives in the United States. Their ISP is demanding they install the ISP's "custom" antivirus. The relative already has a antivirus and firewall application installed that works and causes no problems and is updated and used regularly. However this relative also fears the ISP's antivus suite would cause their performance to decrease (very likely as it is an old machine). Is it legal or even ethical for an ISP to force installation of an antivirus suite they customer does not want or need?

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