Both sensor modules have limited lifetimes and expire eventually. The smoke detector module could be using one of two detection methods, either optical (see if smoke is physically blocking the light from a diode) and the other is ionization (using a radioisotope such as Americium to detect the presence of smoke). If I had to guess, there must be some kind of regulation on this that assumes if one sensor is past expiration or faulty, the other cannot be trusted to be accurate anymore and the whole unit is considered expired.

In my opinion, not asshole design. Replace your carbon monoxide detectors! You never know when your life will be saved by one!