The first two sound almost like a variety of right dislocation, deriving from "Why don't you empty the garbage/be reasonable," rather than arising as tags.  I find when I say them that the dislocated portion is all at a low intonation level, while tags have a rising intonation. 

The problem with sentences like the third and like some of the examples I used is that modals don't have symmetrical polarity.  They don't work the same in the affirmative and the negative.  Some of them do, but enough don't that a traditional Syntactic Structures type of tag derivation doesn't work.  And it is, by the way, one of the oldest transformational rules in syntactic studies.

Herb


 
Chew on these:

Empty the garbage, why don't you?
Be reasonable, why don't you?
Don't tease the cat, ...?? you (will? would? certainly not "do")


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Johanna Rubba   Associate Professor, Linguistics
English Department, California Polytechnic State University
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