The 2 space limit has nothing to do with the manufacturers. It was mandated by the certification authorities: NAR S&T, TRA TMT and CAR MTC for safety reasons.
Motors are certified to provide a certain level of performance in a rocket. If you increase the weight of the motor but you do not add propellant, the Thrust to Weight ratio of the motor drops. For example with 38 mm CTI motor casings, the extra dead weight of the spacer and extra casing length is 50 grams per spacer or 100 grams for 2 spacers or 3.5 oz. For a 1G load, the loaded motor weight increases by 50% from approximately 200 grams in a 1G casing to 300 grams in the 3 G casing. As most rocket that would use a reloadable casing have a minimum weight equal to the loaded weight of a 1 G motor, so by limiting the maximum number of spacers to 2, a maximum weight increase of ~25% would be expected and that would reduce the apogee by a maximum of 20% if no delay adjustment is accounted for, and a typical recovery system should be able to handle the g-shock from a 2-3 second late deployment. The certification authorities were uncomfortable to allow a greater decrease in performance.
Bob Krech, NAR S&T