[john coley]

That is only the case if their equipment is up to standard. Were they using equipment designed to take the required throughput would those factors come into play. Their only course, should they want to avoid their responsibilities in that regard would be to take actual speeds out of the equation, for instance just say "whatever is available in your area". Of course, were they to do that people would vote with their feet and go to an isp who quoted an actual figure. As it stands they must ensure that their equipment is commensurate with the required throughput.
Further to this your argument only holds any water at best if there is no fault. Where there is an acknowledged fault the provider has a duty to repair that fault. If that involves upgrading their equipment that they must do.
John.