Remember, there's a big difference between new OEM, new aftermarket, remanufactured (both OEM and aftermarket) and rebuilt. If you want my opinion, what you have there is aftermarket - either new or remanufactured. Aftermarket parts in general are not finished as well as OEM - hence the rust.
Most shops will not use OEM anything unless they have to because it's so much more expensive. They will also argue that aftermarket parts are just as good as OEM - which is simply not true. I'd ask the shop what kind of parts they used.