Maybe they want cash up front instead of waiting.
That is another option I had thought of, but my post was getting long
It could be legit if:
They want USD
(e.g. don't want to deal with exchanging btc and exchange rates, or don't believe in btc but are willing to try to profit from it)
They want you to pay up front for a lot of mining time in advance
(e.g. because they are afraid the price will go down and/or difficulty will go up in the future -- could also be a scammer trying to run away with lots of funds though)
They are not already set up for mining, and just rent out GPU processing power
(e.g. they don't even know what bitcoin is, and you're the one who has the idea of renting for mining!)
Mining United is a service that rents miners for BTC, which means they take your BTC and give you more BTC back a few days later. This is completely illogical and appears to be an obvious ponzi scam (people are already complaining of not being paid). That's one for your "not legit" list