I would think the guy was really weird and a bit rude to do that. Then again, I err on the side of friendliness and common sense rather than obstinately sticking to the letter of the law with most things. I trust most people to know that they can make it to a shelf and back again in time to not hold me up, so expecting them to check out twice seems silly - if for no other reason than that simply makes someone *else* have to wait (as in, whoever ends up behind them in line the second time), so really, it all comes out in the wash to me. Here the cashiers frequently tell you to run and grab something quickly if you realize you forgot it- and they'll often open another lane while you do so.
I do agree that getting in line for the sole *purpose* of holding a spot, then finishing shopping plain old irks me. I've only seen it a few times, but it's irritating to get stuck behind someone who keeps dashing off, and not moving their cart naturally with the line because they're away.