That's not entirely accurate. the original Wii lost backwards compatibility only because it did no longer have an optical drive that could read Gamecube discs. Gamecube discs are different in two ways: First, they are half-size discs which you generally have to specifically design a drive to support, especially when it is a slot drive. Second, the data on gamecube discs is stored in the reverse direction, so the disc physically spins the "wrong" way in order to be able to read the format. None of these two are the case for the Wii and WiiU. Even without official BC on the latest Wii machines, the hardware inside them still support the execution of gamecube code. All you have to do is hack the console in a way that allows you to feed it with gamecube software.

Even if the WiiU did lose the capacity to read Wii discs (highly unlikely), the console still has the ability to play Wii games (and indeed, gamecube games too if you hack the Wii-mode of the WiiU). The WiiU has no need for physical discs to acquire Wii games, as these can be distributed digitally, so the situation at hand is very different.

In addition to this, I think nintendo consoles are so starved for software that they can't afford to not include bakcwards compatibility.