They're probably selling the opportunity to ad networks. IOW, the site operator 'O' contracts with ad network 'A', and A uses data from the ISP to decide which

They're probably selling the opportunity to ad networks. IOW, the site operator 'O' contracts with ad network 'A', and A uses data from the ISP to decide which ads to serve to which clients via O's site. So the ISP is not necessarily interfering in O's contract with A by replacing ads or such -- at least, it doesn't necessarily work that way. I really don't know, some ISPs have been caught replacing content or injecting ads, but in principle the scheme could be managed without doing so. (My description is from descriptions a few years ago when ISPs were sneakily trying a DPI setup)