Summary: Encryption doesn't solve all security problems. In many cases you need authentication and integrity, not secrecy, and encryption can be the wrong tool for the job. In those cases you need a Message Authentication Code, not encryption.
It's pretty typical to see embedded system designers use encryption to solve security problems. And it's also common for that to be the wrong answer to the real problem.
The issue is that encryption is the wrong tool for the job. What you really want is some combination of authentication (I know the right person sent it) and integrity (I know the contents have not been altered). Encryption isn't the best tool for this -- what you want is a Message Authentication Code (MAC). One way to get this is to compute an appropriately chosen secure hash function and append its results to the message: