ntp, in the 11 page long bug thread, G-Flex claims in post
http://www.bay12games.com/forum/index.php?topic=52685.msg1168626#msg1168626 that Toady One said it in an IRC chat, and posted a snippit from dwarf fortress IRC history.
Frankly, I cannot imagine someone as brilliant as Toady thinking that changing the behavior would stop near instant defense making. If that is what he was after, then wouldn't he have had just given goblins water walking, magma walking, and wall climbing? Then you'd be stuck with fighting it out, because they could cross any moat and pass any wall. Which is what we will eventually get when seiging gets put in properly--- meaning, you won't even get to mega-beasts and other special attackers before you have a "FUN" ending. If he wanted to make dwarf fortress more difficult/challenging early on, without it being always pure FUN, he could have just given goblins the ability to jump 2 or 3 tiles, and therefore make players work harder to create their starting defenses. Or added in minor flying menaces (like eaglemen or batmen) and water ignoring menaces (like frogmen) that occasionally raid your fortress. So I am left presuming he made this change to channelling due to his own belief that "dwarves are too stupid to dig a pit from above", because that's what he is communicating with this change.
If you could remove a ramp UNDER WATER or UNDER MAGMA from above, then this change wouldn't matter ascetically, and therefore the gripe factor would drop to "Now we've got to double designate moats and pits? Sigh!". We already have many processes that require multiple input from us players, such as having to smooth and then engrave a wall or floor, even though engraving should just autosmooth it for us (meaning, it just takes longer to engrave a patch, and if interrupted, the wall would just be "smooth" and await any engraver's attention to be finished).
I do understand that it is easier to designate ramps "from above" and avoid the whole z axis swapping to get a ramp where you want it. But if you are going to be building anything multilevel, you are going to have to swap z axis anyways, and once you can do that, you should be able to work out that the X of dig (or whatever designation you are doing) is where something will be, and just moving up or down the z will allow you to check the placement so everything is lined up.