The encryption/decryption keys for your iMessages are stored with Apple, and are accessible to them. If you have iCloud Backup enabled, Apple–and thus law enforcement–can in fact decrypt them.
A while back, Apple was aiming to transition to store iMessage backups using a more secure method; ie. the same method that some other content such as Health data is stored, which uses encryption keys derived using local info such as your macOS or iOS password. The FBI requested Apple not to make this change, and Apple complied.
Apple runs a great PR game when they talk about privacy, but the reality is nowhere near the same. Some things are fully encrypted so that Apple cannot decrypt, while other things are not safe to store with them. iMessage falls into the latter bucket.