"Who really thought they'd go to US service and hardware companies and ask them to implement backdoors"

To this point has there been any evidence that a company has add in a "backdoor" at the request of the NSA?

The media keeps using the term "backdoor" when they talk about covert access but that doesn't mean that some company added a "NSA Access" feature to their product. It is possible (I would even say probable) that the NSA has exploited unknown vulnerabilities rather than used built in backdoors.

Covert access is only effective if it remains covert. If companies are building in these features then a lot more people know about it and it reduces the likely hood of staying covert.