Another good point. Why would consumers buy a high speed (more expensive) tier if the provider throttled down the services they wanted? Cable companies make more money on the internet than they do pay TV. They want to sell the fast tiers to more customers.
The logical but horrible end to this is the dreaded metered/measured service where you pay for the bandwidth you actually use, similar to what Verizon and other cellular companies do with their data plans. Sure, you can stream to your hearts content from whatever service you want but you are going to pay for the bits you use. Really, though, that is probably the most fair solution.