If the purpose of encryption is to make data unreadable, but a back-door exists that allows anyone with access to it to bypass the encryption, can the data ever really be considered unreadable?
The trick is you don't do that. Ideally you would only want people with a valid warrant would be able to also decrypt the message (bypass the encryption). So the problem is that you want to design a system where this is possible. Perhaps it takes the cooperation of someone from the government and someone from within the company to verify the warrant. Perhaps you have a list of people who need to cryptographically sign the warrant.
There are so many moving parts to manage, infinite possibilities for abuse, and it would require an absolute massive amount of trust in companies with numerous convictions for abuse of precisely that.
They aren't under these types of laws. Typically the government would need to get a warrant first before this is possible. You don't have to let them be able to read your messages by default.