I'd like to explain in lay person's language since this is not a forum of legal professionals. Right to freedom of speech and expression is a universally recognized right and one of the most fundamental rights available to us. However, the said right can be regulated although not completely destroyed. Any rule which completely blocks ur right will be unconstitutional. However, your right can be curtailed by means of reasonable regulation.
For instance, u can't tell a teacher in school that what he's saying is rubbish or senseless and if u do a disciplinary action may be taken like detention. Why ? Cause while attending school u are bound by the school's policy, rules, norms,etc. Similarly, while posting in a forum, you are bound by the regultations imposed by them. As long as said regulations are not totally destructive of ur right to freedom of speech, reasonable limits can be imposed. Merely being non-offensive or not rude may not be good enough. Like here on MFK posts are often deleted just to clean up or when people get into heated arguments then threads are closed. All part of MFK's own set of rules.