In fairness to the Roosters what a lot of people forget is that there are a significant number of SFS members who are Roosters fans, but would not sign up for a Roosters membership simply because they can already attend all those relevant home matches.
There is no doubt hat Souths and Bulldogs would most likely have this issue as well with the ANZ Stadium members program, but it is debatable whether it affects them in the same numbers of the Roosters.
Lets not forget Souths and Bulldogs are both significantly more popular clubs, and Roosters fans are traditionally more likely to be supporters of multiple football codes, hence why an SFS/SCG membership may have more appeal (not having a go at Roosters fans here, but the numbers do indicate that, not that is a negative either).
But yeah in short, the Roosters do have a disadvantage to some other clubs of similar supporter bases, such as Sharks, Panthers, Sea Eagles.