A few points here. Let me first stress that I am not a lawyer, and the below is non-expert speculation on the issues:
Regarding the legality of posting questions to forums from the Official Guide or GMATPrep, I'd wonder whether one could make a case that the reproduction of occasional questions constitutes 'fair use'. Under US copyright law (as quoted in wikipedia), "the fair use of a copyrighted work [...] for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright."
Quoting further, four factors are explicitly identified as considerations to be applied when evaluating whether reproduction of copyright material constitutes 'fair use':
"1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
2. the nature of the copyrighted work;
3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
4. the effect of the use upon the potential market for or value of the copyrighted work."
As pointed out in the GMAC memorandum requesting default judgment in the case, ScoreTop "have permitted users to post links that provide copies of entire GMAC test preparation books." That is, they reproduced copyrighted work in its entirety (viz item 3), and presumably damaged the market for the original work (item 4). In addition, ScoreTop charged money for VIP access, and was thus of a commercial nature (item 1). In these respects, gmatclub is different from ScoreTop.
Regardless of whether reproducing GMATPrep or OG questions does constitute a copyright violation, there is the separate question of whether GMAC would have any interest in pursuing legal action.
And to address a couple of factual points:
GMAC pursued action against ScoreTop on three grounds- copyright infringement, contributory infringement (for encouraging others to reproduce copyrighted material) and tortious interference (for violation of the non-disclosure agreement agreed to prior to the test). They did not merely pursue a case for copyright violation. While no damages were awarded based on the tortious interference claim, the judicial opinion did mention that such damages would be 'duplicative' of the damages for copyright infringement. By my reading, the secure nature of the GMAT test also contributed to the decision of the amount of the damages awarded.
Finally, GMAC is a non-profit, not a business.