There is no other way for this to work that won't result in an absolutely massive number of people losing their data permanently who had no idea their drive was encrypted. Well there is, leave BitLocker disabled by default and the drive unencrypted. Now the police don't even have to ask!

With this scheme the drive is recoverable by the user and unreadable to everyone except you, Microsoft, and the police. Surely that's a massive improvement over sitting in plaintext readable by the world. The people who are prepared to do proper key management will know how to do it themselves.

Apple does the same thing with FileVault when you set up with your iCloud account where, again, previously your disk was just left unencrypted.