Given that the business is merely renting the modem/router/AP device from the ISP, the only theoretical downside to the business is the extra electricity used by that functionality. And for quite a few businesses, the zero-effort zero-liability hotspot for customers is a pretty big upside.
If you want thorough control over your network, then of course you won't rent a modem from the ISP and you'll install a separate router of your choice and decide for yourself whether to operate a hotspot. But for the majority of customers, none of that is worth thinking about and the ISPs are actually providing sensible defaults.
