From Anchor Gaming's perspective, the no sell-no trade rule maximizes the play on their machines, which is good for them. At one time, they participated in the revenues from each machine, though not sure if that is still the case. If a new series of 5 SS's comes out, the collector will tend to play until he/she has one of each. If they trade duplicates at the cashier window, you only need any 5 SS's to get your new set. If they don't trade, you need to play more coin-in to get all 5 different out of the hopper. Even if you redeem the extra strikes for cash, it costs more than the $10 redemption value to obtain the dupes (on average).

FYI, if a machine is loaded with an equal number of 5 different SS's, the odds of getting all five without getting a duplicate: 25 to 1.