allow bitcoin users to claim a share of any pre-mine in direct proportion to the percentage of bitcoins market cap they control.
If you are a bitcoin user, you can use your bitcoin private keys to claim a percentage of æther exactly equal to the percentage of the bitcoin market cap you controlled at the time of nucleus creation.
If I understood correctly, the more bitcoin you have, the more premine you'll get. In other words, the rich will get richer and Satoshi Nakamoto will get the lion's share of any altcoin premine.
Surely you had something else in mind, didn't you?
It's not "getting richer." The way spin-offs work economically is that the value of the spin-off is already included in the value of the parent. What happens is that the existing value is split between the new ex-spin off parent and the new instrument. How much accrues to each depend on the value the market attributes to each.