Why does environment AP have diminished returns in cascade?

If you make cascades of multiple chains the damage at each additional match is reduced to 75%, so the last matches of a long cascade do pitiful damage (see here viewtopic.php?f=9&t=572). However you get the full AP for those color matches even if a 4-match is doing 10 damage. However, environmental AP does not get full credit, an environment match at the end of cascade can give you hardly any AP. I have not done the math, but I would assume that it also is 70% after each match. Is this by design? It seems like it should get full AP like the colors since it is not doing damage.