It's an interesting subject, because it's hard to reconcile it effectively. Yes, of course teams should play their best XI in every game, and play with strategy and tactics designed to win that game. That's how it should be in a perfect world. I don't think anyone can disagree with that here.
But also, as with the example of Huddersfield playing a weaker side, you absolutely cannot fault them. They have a real chance of gaining promotion to the Premier League, and resting certain key players for these last few league games will be vital for them to gain the edge in their play-off games. And why not? They need to think about themselves. Why should they care about other clubs? The idea of a football team being purposefully disadvantaged by their own management for the sake of another club is ridiculous.
Also - Define "weaker". Where do you draw the line? If we're going to have a rule about this, we'd have to have a similar thing to what we got in the Checkatrade this season, with a specific limit of squad changes between games. Which is obviously chaotic, as demonstrated in practice. There is no answer to this, IMO. It's a loophole.