If you get 4 common from a common chest, you can reroll without a warning, because thats the minimal reward and you have a chance to get a rare in the next roll.
When you get 1 rare and 3 common from a rare chest andyou reroll, you get a warning… why? This is the minimal just like in the previous case, and you are very likely get another rare, or even epic/legendary, worst case you get the same rarity.
Same with the other higher tier chests (epic, legendary).
This is annoying when i open 20+ rare chests and i use the reroll more.