Maybe the legacy airlines didn't want to set any precedents about one person earning from flying activity of several people because they saw a slippery slope to a reclaim program if they violated the one-person-one-account structure. They also wanted to make money on transferring miles between family members. @:-) And they generally didn't think a family of four they'd see once or twice a year deserved much of anything (though they won't say that).

Not that I'm defending any of that. Frontier's idea might catch on but I think they're limiting it and paving the way for confusion by requiring the pool head to either be an elite or a cardholder.