Round here the cable folk tend to scan their network for activity on port 80. I have some friends who have been discovered...the provider's reaction to the discovery ranges from a polit "ya gotta stop that" to a visit from the technician for a disconnect at the pole.
If people carry on doing it they may just firewall the whole lot so no inbound FTP or web connections? I have seen ISP's who block outbound SMTP connections forcing people to use their outbound SMTP server in an attempt to try and reduce or control SPAM sent through their network.
As with many providers, they don't get excited until something crops up that might disturb their revenue stream.
The problem is exactly that - they don't want to sell someone a £30-40 per month service that is costing then £200-300 in bandwidth.
Thanks...