Apple and Microsoft have provided source code reviews to customers of a large enough size. The federal government, for example, reviewed sections of Windows source code before installing it on things like Navy ships, NSA computers, etc. Of course, this does not do us regular schmoes much good.
That said, this problem is a much better problem to manage than the one declan is talking about. Imagine if you wanted to use open source encryption to secure your information but were legally prohibited from doing so. That's what law enforcement is talking about now (and as declan points out: again).
It's un-enforceable as well unless, ultimately you'd end up needing to police the consumer.
This sort of misguided action in today's technology culture will just result in an exodus of hardware manufacturers + technology service providers and shrinking demand & trust for US based technology.
This is already happening to some extent, illustrated by the boom in ASPs & Cloud computing occurring in Germany, elsewhere in Europe and to a lesser extent in Switzerland.
That said, this problem is a much better problem to manage than the one declan is talking about. Imagine if you wanted to use open source encryption to secure your information but were legally prohibited from doing so. That's what law enforcement is talking about now (and as declan points out: again).