I do see what you mean, but I'm convinced they're not meant to make sense - it's so you can be got at the subsequent board of inquiry - that is, some civil servant who is responsible for that aspect of aviation doesn't have his pension snatched because you broke the law.
It's the same reasoning behind the banning of night flying in single-engined helicopters with passengers in UK - you can still fly with pilot only, so it's obvious that the absence of passengers doesn't make the process any safer - it just covers someone's backside.
However, that particular phrase says "operator", the definition of which does not include the word "pilot", so, unless you are also the owner or person leasing it you should be safe from hassle if you get a drunk on board.
But what are "reasonable grounds" anyway?
What prompted your query, BTW - were you just reading CARs one day in the bath (as one does) and come across it, or did you get an incident?
cheers
Phil