As a legal matter, I don't believe there are any precedents for a vendor being forced to put a backdoor into a product they are distributing to others to be used in a place outside of the possession of the vendor.[1]
While still vulnerable to all sorts of technical problems as described in TFA, a website that treats the encryption/decryption code as a product they distribute to customers may be[2] subpoena-proof. Just because there are actions I can take to help the police gather evidence doesn't mean I can be compelled to do them. Turning over evidence in my possession is entirely different.
[1] I would love to hear one example of a vendor being legally compelled to backdoor a product. Things that definitely are not examples but people often say are examples include: Lavabit, Clipper, RSA's BSAFE, Hushmail, the NSA modifying hardware on their own, CALEA.
I would also, at this point, accept non-Internet analogues. Has a car dealer ever been required to install a GPS tag in a car they sell to a customer, for example?
[2] I would not recommend anyone volunteering to be the test case.
edit took out mistake about BC versus Canada
EDIT To be clear, I'm questioning whether Hushmail was ever actually required to give a backdoored version of their code to anyone. This is opposed to having to give over information that was or would be within their servers at some point, even if Hushmail had to modify their systems to keep it.