Actually it doesn't say United is necessarily buying the aircraft, it said "we, or one of our partners will convert to firm orders"...Skywest is the only company to my knowledge that has firm orders and are also a united express partner.


Here it is:
"We announced we will add 10 Embraer E175 aircraft to the United Express fleet, offering more customers greater comfort and convenience during their inflight experience. We, or one of our express partners, expect to convert 18 more E175 aircraft to firm orders in the near term.

We anticipate deliveries of the aircraft will begin next year and continue through the summer of 2017. The new aircraft will replace older, less-efficient 50-seat regional jets and are in addition to the 125 E175s whose deliveries began in 2014.

"Our customers tell us that they are thrilled with these modern, fuel-efficient aircraft," said Fleet VP Ron Baur. "The E175s are a great addition to the regional fleet and further align the United Express inflight experience with our mainline service."

The E175s will offer 12 seats in United First and 64 seats in United Economy, including 16 extra-legroom United Economy Plus seats. The aircraft also offer more personal space for customers, with wider seats and aisles than other regional aircraft; a power outlet at each United First seat; and large overhead bins that can accommodate standard-size carry-on bags.