There are legitimate reasons for asymetry on DSL and cable
On DSL upstream and downstream have to be given seperate frequency slices out of the limited bandwidth available on a typical phone pair (which lets not forget was only designed to carry voiceband). So you have to tradeoff upstream speed and downstream speed and for most users it makes more sense to tradeoff towards downstream. Having said that I do think it's scandalous that symetric services are insanely expensive compared to asymetric ones of comparable total bandwidth.
On cable the technical reasons are even greater, cable networks are designed for broadcasting TV with a high power transmitter broadcasting through the high-loss (due to the splitting/padding) network to a lot of receivers. Upstream traffic is going against the flow which means it has a lower acceptable transmit power and a lot more interference present at the receiver.
On the other hand with fiber the only reason for the asymetry is artifical crippling (making it harder to use P2P, run servers etc)