I get where you're coming from, but I do like that the easy/normal treasures drop on Hard/crushing because one Hard and crushing difficulties, the drop more often. This way you can get way more easy and normal treasures than playing on Easy or normal.
You can even test it out, play on easy and then crushing. You'll see a major difference in treasure drops. Even though I do agree it would be nice if they do only drop hard/crushing treasures on the hard/crushing difficulties, I do like getting the easier treasures because you may get lucky and get some new easy/normal treasure.
Even if they did just add hard/crushing treasures to the hard/crushing difficulties, it'll be like having silver or gold Treasure hunter on Competitive and get the same treasures over, and over. I don't know about you, but I think I would like to get a better veriety rather than the same thing over and over.