It's a conflict on interest, no matter what your job title is. For example, UPS charges higher ground shipping rates than RPS (a.k.a. FedEx Ground). Yet many consumers that switch to RPS return to UPS... why? Because UPS is more reliable. You working & doing a great job for RPS would actually be hurting UPS. The same applies to other air jobs -- Northwest, United, American, etc. are all big compeitors to UPS's same-day freight shipping, which is a bigger money maker.
It shouldn't take a genius to figure out that a UPS employee working for FedEx isn't exactly dedicated to his/her job (although he/she may excel at it)... if you work for UPS/FedEx/etc. and take up a job with a competitor, it's best to discuss it first with managment (they may OK
you working for, say, Continental -- but get it in writing!).