A person's everyday use of a broadband ISP e-mail address is only logical if they have a somewhat permanent consumer relationship with their broadband ISP.
Possible criteria for such security:
1) The user will remain at their current place of residence for an extended (multi-year) period of time. This excludes renters and other non-homeowners. It also excludes people without stable or established careers, as they could be required to relocate.
2) The user will pay their bill on time. (Otherwise, their e-mail address will go away.)
Additionally, having a broadband ISP excludes the rural American demographic.
People fitting in both 1) and 2) will be more likely to have positive credit scores for obvious reasons.
While I agree there's likely a correlation to the two criteria you mention, there's a third which is think is more at the core.
That is that for a ton of ISP subscribers the concept that email doesn't need to be coupled with their internet service is not something they understand. For these people the idea that they'll need to change their email address when they switch ISPs is taken as a given. Just as if they were to move they would need to update their mailing address.
Possible criteria for such security:
1) The user will remain at their current place of residence for an extended (multi-year) period of time. This excludes renters and other non-homeowners. It also excludes people without stable or established careers, as they could be required to relocate.
2) The user will pay their bill on time. (Otherwise, their e-mail address will go away.)
Additionally, having a broadband ISP excludes the rural American demographic.
People fitting in both 1) and 2) will be more likely to have positive credit scores for obvious reasons.