The same reason it would be nice to know if a traffic-stopping car accident occurred: I would use it for shortening commute times. Why should I wait in line to tell an officer that I'm not doing anything illegal when I haven't been doing anything illegal?
Whatever. There's been more than one time where I was coming home from work at night and after the commute found a DUI checkpoint stationed on the main road right in front of my street. No way I was going to sit through the bullshit waiting just to "prove my innocence" as you seem to imply we all should.
Just like if it were a major, road-blocking accident, the answer for me (and I'm sure many people who were not sober) was U-turn and reroute. There's no reason why law abiding citizens should have to be inconvenienced by these dubiously effective law enforcement efforts, and if there's a tool based on protected free speech that allows us to avoid them, so much the better. LE can catch drunks in other more effective ways that don't inconvenience the law abiding citizenry.
Slightly off topic but still relevant: There's actually legislation in some states that would permit the police to chase you if they see you purposefully trying to evade a DUI checkpoint. It differs by each state(or municipality?), but New Mexico seems to have enacted that:
Nobody is suggesting that drivers should be free to police themselves as far as the legality of the circumstances of their driving. This is simply a matter of freedom of speech -- if you see a police car doing radar on your drive home, and call your friend (after pulling over ;) ) who you know will be driving this way, to warn him, are you breaking any law?
The principle being defended here is that you are free to share that information under the 1st amendment, however the method of sharing may be performed. It's the reason being a member of the KKK or the Westboro Baptist Church and saying hateful things doesn't automatically make you a criminal. Whether you agree with the speech or not shouldn't affect it's possibility to be spoken.
