The OP asked what tv channels one might get along with their internet only cable service.
As has been said it varies by system, and there is no hard and fast reason why. My neighbor works for Cox and Cost is the reason they don't go out and put a trap on every internet users line to block the locals and the government access channels. Cox still has some analog and hybrid analog/digital systems, perhaps that changes if they every get to fully digital.
As to why those channels are unencrypted to begin with, that is because of the fcc ruling for tv service. So they are intertwined.
Most people go to one of the many Cox stores and just pick up a modem or set top and self install. Why spend more money to roll a truck just to install a trap and then have to roll it again to remove the trap, if that sub decides they want tv or a different individual moves into that location and wants tv.