I have a feeling in the next few years once DOCSIS 3.0 gets fully rolled out with all the hardware upgrades and so forth, that those caps will either raise significantly or go away altogether. Comcast knows they're silly, the industry knows they're silly, and consumers know they're silly and they're not fooling anyone. Once 8 by 4 channel upgrades are the norm and with them already researching more channel bonding, the per node bandwidth will start to increase exponentially from where it is. Right now most systems are only using a 4 by 1 channel bonding (down/up), meaning many systems are only capable of roughly 160Mbps/43Mbps (approx) per node. Those 50-100Mbps+ tiers are NOT meant for anything but a publicity stunt against the likes of Verizon and Fios, hence one of the reasons the price is ridiculous and capped at that. Do the math, two people downloading could quite literally rob the entire nodes bandwidth potential which means either caps or traffic shaping, or both.
One of the reasons for the caps is quite possibly to stem piracy, kinda like how the Feds are using child porn as a reason to rape our rights online, ISP's are using that as another possible reasoning assuming that only those downloading more than that aren't tech junkies but rather evil pirate criminals. It wont last forever. @Home tried the whole connection cap instead and failed back then when Optium Online was the goto cable provider that everyone wished was in their neighborhood.