This is, I think, the key feature. While the US does consume a bunch of planes, it is by no means the only market. What Delta did for Bombardier was to give them a 'light house' customer[1] to show the world that this plane was worth considering.
[1] The 'light house customer' metaphor was a favorite of Chris Bennet's at NetApp. They were a customer that if they adopted your product, others in the same business would see your product as a "safe harbor" and would be willing to buy your stuff too. The unwillingness to be 'first' in an industry where one plane represents a big chunk of capital is strong.
I figured that Swissair was that lighthouse customer. They've had very positive feedback for the C-series, which is quite impressive for a brand new model. Honestly I doubt Delta would've even considered the C-series if they were to be the first major customer.
IMO the big reason that Boeing is going after BBD is not the CS100, but the prospect of a CS500 which would compete directly with the antiquated 737. Boeing has, for now, left Embraer alone because the E2 can't be stretched as much as the C-series.
The hypocrisy is strong with Boeing though. Boeing's received over $8 billion in tax breaks (errrr subsidies) from the state of Washington alone.
[1] The 'light house customer' metaphor was a favorite of Chris Bennet's at NetApp. They were a customer that if they adopted your product, others in the same business would see your product as a "safe harbor" and would be willing to buy your stuff too. The unwillingness to be 'first' in an industry where one plane represents a big chunk of capital is strong.