This discussion about rules is becoming interesting, even if it sounds not.
A lawyer would tell you that in a modern country, the Law comes from:
1/ the constitution of this country
2/ treaties made with other countries (aka European Union), which must be compatible with the constitution above
3/ the internal laws of the country
4/ the administratives rules in the country
5/ the custom
6/ the jurisprudence coming from particuliar cases that have been judged.
and of course, every point must obey the previous point in that order.
So, where would a forum rule take place in this scheme? In a 7th point, that would have to conform to all previous rules.
You understand then that a forum rule should only be some general recommandations about politness, norm of writing on the forum, subject of the forum, and not a valour system that could prevent any one to sell/trade anything just as he want. Free capitalism is a law in western countries as long as it is not forbidden gears.
In conclusion, a public forum can not be under a super rule and the "if you don't like the rule, do not suscribe" is irrelevant.