I don't think the 4-hour SLA customers subsidize the 72-hour ones. It's more about managing the volume of support. If know you won't get an until 3 days after, you will google obvious things yourself, and only contact support if you can't get the answer otherwise. But shorter SLAs, let alone phone support, encourages a particular kind of customers to just copy-paste any error message they encounter (that may not necessarily come from your product) and expect support to untangle it. Been there, seen that.