Because.

I like it. It should probably a replacement effect, though. Other than the fact that this is strictly better than Forest, it could actually be printed!

Hmmm. It's not really actually much better than Forest. Presumably the intention is for this to override "put onto the battlefield tapped", but actually virtually none of those apply to generic land cards: only Loam Dweller and Primeval Titan as far as I can tell. And, well, you've probably got better things to search up with the Titan ;)

I appreciate the name. I've previously had the Magic card generator create lands with ": Add " which I've named "Is Land", but I never thought of this one :)

I was mainly annoyed at the pre-eminence of "~ETBs tapped" as a fix for funny-lands.

And, I guess you are allowed to make cards slightly better than vanilla. ­Snow-Covered Forest, for 1example.

this would, presumably, be really quite nasty if it were ALSO basic, since then you could Kodama's it untapped (and other similar things)

Oh yeah, this would be stupidly strong if it were either "Basic Land" or "Land - Forest". (This is all assuming that text works, of course. I'm not sure whether or not it does.)

I don't think the text works either. "When this land enters the battlefield tapped, untap it." works fine, but might kill the joke.

The text actually works (well, not the minimalist ":", but that's another story). ETB tapped is just a replacement effect, like "ETB with four +1/+1 counters" or "ETB as a copy of a creature of your choice", and so then would this text. With multiple replacements applying to the same thing you control, you can arrange them in the order you like, to get the desired result.