We had this debate just the other week, where someone was outraged that tenants were being approached directly by the EC. My view at the time was, and still is, that as a tenant I’d rather deal with complaints myself than be dobbed in to my landlord who might then kick me out.
In this case, I think you treat the tenants as adults and tell them that they are in breach and give them an opportunity to mend their ways (they won’t be the first residents – tenants or owners – to not know what the rules are).
But if they persist, you certainly should contact the owners and explain that their tenants are in breach of the by-laws, and therefore in breach of their lease and ask what they plan to do about it. If they say ‘nothing’ then, sure, send them a Notice To Comply too.
I think the big difference is that the tenants are in breach of the by-laws whereas lax landlords are in breach of the strata Act (Section 117, I think) by permitting their tenants to breach the by-laws.
It’s not as simple as dragging the landlords in on the same Notice To Comply – nor do I think it should be. You can’t hold people responsible for other people’s behaviour – all you can do make them accountable when there’s something they could have done but haven’t.