I'm not even sure where the official rules end and where the house rules begin and a quick Google search didn't find anything for me either, but I think that's stupid (no +2 on a +2) and it runs counter to how the rest of the game is played (unless I'm also going off of other house rules, I'm not clear on what the "official" rules are).

So the main mechanic of the UNO is matching cards, whether it's by text/symbol, or by color. I can play a green 2 on a red 2, because the text/number/symbol matches. I can also play a blue 'Skip' on a yellow 'Skip' or a green 'Reverse' on a yellow 'Reverse' or I can match them up by color, the whole point is that you're matching things up because that's how the color cards work. The black cards Wild and Draw 4 work a little differently, that's understood (for the most part), those you can't play unless you have no other option.

Why then is it that the 'Draw 2' cards are given a special place, why even have color versions of those cards in the first place unless you're trying to confuse players? If they had wanted them to behave differently, they should've made them black-bordered and/or multi-colored like the Wild and Draw 4 cards, that would let people know, "Oh hey, these have different rules." Instead, they're made to look like all the other color cards and then the UNO Industrial Complex says, "No, you can't actually use these the same ways as every other card, you're an idiot for thinking that." The reason so many people "screw up" this rule is because they're playing the game consistently and they're applying the main mechanic as it should be applied.