Good points. In response:
From what I've read, handheld jammers have very limited range (15-20 feet). They're designed to "knock out" the loudmouth businessman who is next to you (screaming into his phone in the cab line at the airport), or the gum-snapping woman behind you (the one with the screechy, whiny voice) who is talking to her sister in law about American Idol in the supermarket checkout line...not everybody on the block.
So is it possible they could conflict with an emergency call? Yes, but unlikely, since the user is in range of the person using the cell and uses the jammer because they are painfully aware of the call content. No, it's not perfect (jamming could conceivably block an "important" incoming call), but I think this context is important to understand.
Now, get into the legalities, and you enter a massive grey area. At minimum, U.S. law allows people to block cell phone signals on their own property (which is why plenty of restaurants and theatres already do this).
And while the FCC states that manufacturing, selling or owning a portable jammer is illegal, the agency admits it has never seized a single jammer or prosecuted an operator . This is likely due to the fact that (a) catching someone with/using a small portable one is virtually impossible and (b) cell phone service is still so spotty that when a cell phone doesn't work, the first thing people think isn't "I'm being jammed."' In other words, there's no public outcry, no CNN investigation, so this sneaky and obscure "don't get mad, get even" practice is still way, way under the radar screen - and may likely stay there forever.