It used to be the case that a landline was required to get DSL. I don't know if it still is, but that would be a reasonable explanation.
DSL is available without telephone service, at least with Verizon in Pennsylvania. It is called "dry loop DSL." 3 Mbps / 768 kbps is costing me more than the price they are advertising for 50 Mbps / 50 Mbps FiOS (which I can't get) without contract. So it exists but it sucks, and I'm not sure you actually save much (or anything) by getting DSL without telephone service.
I wonder how many just have it because it comes with their cable/satellite TV bundle? Satellite companies used to really push that as required (and charged an extra fee if you didn't have it) because their boxes used it to phone home.