This is a great point. I especially agree with your final sentence. I'm confused by your earlier example - I do think people can use 'artistic license' as an excuse or coverup to a mistake. I don't think that's the same as having an explanation for decision - even if that decision detracts from a piece. Am I misunderstanding the link you're making between the opinion and the example?
Of course having an explanation for a decision is all well and good and I'd never put a stopper on that. However, I think a lot of people might rather find a justification for a mistake they've made rather than trying to fix it, whether consciously or not - it's basic cognitive bias, we tend to have our opinions before we find rational explanations for them (and what can seem like a conscious artistic choice can subconsciously be the result of the fear of having to go back to the piece and revise it for hours on end!). I know I'm being very cynical here and probably unfair to a whole range of writers who would wipe the floor with me, but it certainly doesn't hurt to at least be conscious that there's always an element of self-justification in any conscious decision we make. Maybe that's why you'd in 'the olden days' practice imitation for years and years before trying something new - to make sure you have a 100% grasp on form before innovation.
The Society of Classical Poets publishes formal poetry. The closest thing to free verse is the poetry that follows fixed syllable counts rather than meter.