Pretty much every example you've given shows how easily they could do it, yet they haven't. Hmmmmm. Wonder why? Guess they don't give a shit.
Wii did have a little more developped system than GC. Yes, it's not even close to WiiU/3DS, but these checks could have been done. You're right about the bans, but they could perfectly made a sanity check during boot time by altering bootloader/homemenu. One time to do the check would have been enough, and they could make the system unrecoverable (except with BootMii on Boot2) easily.
There are technical (easy) ways to achieve that, so this is definitly not the issue here.
This is just NOT the way Nintendo works, and the lack of 3DS bans for CFW users is just another proof. They try to prevent the use of Homebrews/CFWs, NOT to punish users (which is IMO a way better thing to do, but pretty inefficient as we can see).
I am sure bricking the console is out of question for nintendo. Not even sony will employ checks to basically brick your console if you have modified it and it even is illegal in some parts of the world AFAIK as modifying your system is legal more or less in most places in the world. AT LEAST MODIFYING ITSELF, not all things you can do with modified system tho.
Since Wii lacked any robust online services there was nothing to ban, and like said online "service" was per game as the game itself used it's own setup and all.
Even then any of these checks, I already said this, would been an band aid that hackers could rip off easily by just patching the checks out themselves which they could do, easily. Whole CFW on 3DS is based on patching out nintendo's signature checks and 3DS security is tons more advanced than that of the Wii.
IN HYPOTHETICALS:
Possible? Maybe?
Illegal if the outcome is bricked console? Maybe
Ban people from online? How exactly
And what's the point? Hackers will remove this anyway.
Like said; already, ALSO (making me repeat so much stuff here) nintendo DID focus on what was prudent; they did patch out banner exploit for the wii and few other things like that, though game specific exploits could not be patched out since Wii was wii we discussed how this works already, and there was no system to patch the games either. Letterbomb is last thing Nintendo seemed to do nothing about on the Wii itself but weirdly enough addressed it in Wii U, making hacking of the Wii U's virtual Wii possible with game specific exploits only.