This is the very issue I do not understand about UA code-shares. I was on the phone a couple of evenings ago with a nice UA rep at M+. I asked her about mileage accrual on code-shares. This is what I found out:

- When travelling code-share with a UA flight number on another airline. The operating airline's mileage accrual rules apply. For example, if you fly on ANA with a UA ticket, but at a discounted fare you would "only" receive 70% miles. LH is the big exception to the above.

- When travelling on a code-share with a UA flight number on a non-*A airline (for example, Saudia, Emirates, etc.), your miles do not count towards Premier status! Crazy!

From the above, it seems that there is absolutely no incentive to buy your ticket from UA instead of the operating airline.