To be fair to them it isn't unusual for the failing calliper to also necessitate a replacement ECU also, the ECU is in reality a high current driver for the calipers, which over time does suffer degradation. When the motor fails and gets gunked up it draws more current than designed for and that could affect the ECU itself, which might not manifest itself until your replace the caliper motor.
On my old car, I had to swap both motors and the ECU to get rid of the light, and that was following the correct procedure
So maybe not all their fault - if it is any consolation, the ECU wasn't the most expensive of parts from memory