They don't count the "shaped/throttled" sites against your data plan limits, so I can see some people liking it.
Some throttling by default for video actually is sort of for your benefit, at least indirectly, although I don't know if carriers are throttling more than that amount.
A surprisingly large number of people nowadays watch movies on their phones or small tablets. On screens that size at the distances they typically hold them from their eyes 4K and usually 1080p won't offer any visible improvement over 720p. On some devices even 720p probably won't offer an improvement over 480p.
It is to everyone's benefit if other people on the network don't stream at a higher resolution than is necessary to reach the limit of what is visible. Since most people aren't going to change any settings from the default, defaulting to low speed streams maximizes benefits.