Staying on the correct side of a yellow line is not a guideline, it is the LAW. It is illegal to drive on the wrong side of the road.
That's true, but it does not stop people from doing it.
Conforming to Celebrity's guidelines (in their own words, when I spoke to their rep on the phone) for what to wear on their ships IS a guideline, NOT the LAW. It is NOT illegal to wear blue jeans to the after dinner show, or EVEN in the dining room, believe it or not!!
Sure, you won't get arrested, or get a citation, for violating the dress code. Nonetheless, the norms of social etiquette dictate that we all must follow such guidelines. In fact, social etiquette dictates the use of terms such as "suggest" and "guideline" rather than "require" and "standard" -- but social etiquette also dictates that such softer terms carry the same expectation of conformance on the part of participants in the associated function or event. If you have doubts about this, go to your local library and read what any book on social etiquette says about the matter.