Probably they cannot be repaired. Think of it like this, the shipping there and back will cost money. The sensors are probably made in China, so they don't actually have parts in the country. Could they import them? Sure, but then they need to pay someone to receive the package, have a system in place to track the repair, have someone repair them, pack them up and give them back to the shipping department. By the time all that was done, if would be cheaper and easier to just buy new ones.
Mobiles phones cost hundreds of dollars and they are viewed as disposable items. Where do you think that leaves a $40 to $50 sensor?
What would be better is to make them modular and allow the housing to be reused and new internals put in. Then they could just make the internals as a part that can be ordered. Chances are, alarm people would rather just pull a new one out of the box. By the time they bill for labor and the sensor, you're nearing the price of a new sensor.