The difference between MC and Derek being members of the DiKs and Caleb being a member of the Alphas is that the Alphas are under Administrative supervision, so any rule breaking might result in their frat being dissolved.
The agreement was that the HoTs will plan parties with both fraternities.
The DiKs planned the xmas party by themselves, therefore they broke no rule. Just because half the HoTs decided to show up doesn't mean the DiKs broke the rules. Moreso, even if I'd agree that someone broke the rules, it would be the HoTs who showed up, since they agreed to attend the Alpha party. (To be clear I'm not saying that

)