Illogical ETS requirement for existing non-res. buildings?

The 25-foot boundary seems to be the norm for this kind of situation -- not at all unique to LEED. I would guess that LEED borrowed it from some regulation or recommendation about avoiding secondhand smoke. It does seem a bit arbitrary to enforce the boundary only "on property," but what can LEED, or the building owner, do about public sidewalks? The credit simply enforces the boundary where it can, and leaves the public domain to public rules. The engineering situation is an interesting idea, and I would recommend submitting a CIR to GBCI if you want an official opinion on this. There are obvious reasons why this solution would not be preferable, among them energy use, and the high risk that it would break down or not perform as intended in all circumstances (what ventilation system doesn't have these problems?) and so would not be a reliable protection for occupants.