That makes no sense at all considering that the general codes would have been used not only by the person who originally recieved them but also by others. Using what you've stated, it is clear that sales due to coupon use would have been much higher if general codes would have been available in those months that only single use codes went out.
If, on the other hand, you are comparing months where both single use and general codes went out, including those single use 10% off website codes then of course the single use codes produced more sales because many people figured out how to use those codes on multiple purchases and people will always use the higher dollar saving option when presented with a choice; $20 will lose out to $25 off or 10% off codes.
Sideshow doesn't have to offer the $20 codes but if they feel the need to defend getting rid of them then they should take all relevant variables into account before making general statements, in my opinion.
For me, the $20 codes are not going to be a deal breaker for things that I may really want but dropping them has helped me to break the monthly order chain that I had gotten into. It is amazing how easy it gets to just say no to such discretionary purchases...the first no is the hardest but then it gets easy. So, thanks Sideshow. ☺