Here's the point: Business usage (phone, cable, whatever) CAN be more costly to the provider because these users will scream louder and demand quicker restoration of service when something goes wrong (line failure due to snowstorm, flooding, you name it). They also threaten to sue for lost business revenues due to the company's failure to restore said service in what they think is a timely manner. Residential customers don't bring that baggage.
So, they don't really care if you USE the line for business, because you won't be able to file suit as in the case above -- according to the TOS you weren't supposed to be using it for that purpose anyway. BUT, if you want them to treat your service as an essential component of running your business, you have to pay business rates...which is not wholly unfair IMHO.