Yeah, I totally agree. It's really against the spirit of the game. However, it's not that smart. You could post a very weak defence and most probably lose less points than not posting a defence at all.
A streamer has popularized this (after making a video about not posting a defence is cheating and deleting it) and had success with it. I think it started as a tantrum defence against GP differences.
So, I don't really think it's a loophole, but it prevents players having a 'proper' GAC and therefore as you stated is 'against the spirit of the game'.
As much as I love modern GAC, I think CG should look at it 'with the spirit of the game' in mind, and try to give everybody a good game every match up.