Last time I opened an account with Comcast they required your social security number. Same with an AT&T cell plan.

Strictly speaking they require /a/ Social Security Number, not necessarily /your/ SSN.

When I signed up for my most recent cellular service, I hesitated at giving my information to the guy in the store. I told him that since it was a pre-paid account I wasn't asking for credit, so there's no need for him to have my information.

He was OK with that, and pulled out an ID card the store keeps in a drawer for just such occasions.

Salespeople have a sales quota, not an enforce-the-rules quota.

Same when I visited Germany, every phone account needs your name and address and passport number to prevent spam, I bought a prepaid card, since I don't live there I don't have an address, so he used the address of the store. Still needed a passport though.