The real question here would do they have a legal monopoly in these areas? Obviously in that case they're obligated to provide service regardless of whether or not that service is profitable in each and every case. If not, they might still be obligated if they accepted government subsidies to provide service.
I suppose they could also use profits from more profitable areas to subsidize unprofitable services but that might not go over all that well with the customers in the more profitable areas. There might be a good argument for a farm or other business which by necessity must be in the boondocks to get Internet service which is subsidized by someone else but that argument falls apart for people living in McMansions in the exurbs. Honestly, same thing with roads and other government services to these types of developments. If the people who want to live there can't afford the true cost of goods and services then they can't afford to live there. So yeah, you can argue for subsidized services to low density areas sometimes, but not for low density residential. NYC or Chicago obviously need farms elsewhere to feed people, but the surrounding suburban or exurban developments offer no benefits whatsoever, except possibly to those who choose to live there.
