There are a few reasons why it's not a good activity to maintain or encourage. First, it's a slippery slope and people easily find reasons to justify one activity because of their misconceptions that they're good at a similar one. It's not far to go from "I only text at lights" to "I can text coasting up to a light" to actually driving and texting.

I can't seriously treat grown people like that. I don't appreciate being treated like an idiot. People should be given the benefit of the doubt until proven otherwise.

Second, there are lots of reasons why everyone needs to be paying attention even while stopped. A lot of people think in terms of "well I'm not going to cause an accident", but you really have to think of all the idiots around you doing all sorts of stupid, distracting things in their cars that will. You need to keep your head up so you see the dummy coming at you the wrong way, the guy behind you that's texting and doesn't see that you're stopped, the pedestrian that steps out on your green for some reason. Everyone around you is being an idiot in their cars in some way - wouldn't you want to be alert to that at all times and, at the very least, not contribute to it?

I'd LOVE to see some stats to prove any of this fearmongering. Are there reasons? Yeah. But they aren't significant.

And to your question, not enough to warrant involving the law.

Finally, attention switching is a cognitively demanding activity that has a latency period that impacts each switched to or from activity. Meaning, that when someone is gearing up to do one activity, their performance on the current activity drops. When they switch back, it takes time for them to ramp back up to the new activity. I'm not familiar enough with the distracted driving literature to know if this has been demonstrated with texting and driving attention, but it's been well documented in loads of other tasks. So it's possible that when you're mentally preparing the text before your stop your awareness/performance of driving decreases, then after you text and start moving again there's some time before your driving awareness/performance is back at 100. It's better to not interrupt that attention and situational awareness environment at all with a distraction as dissimilar to driving as texting.

Again, I can't treat people that way. I don't like being treated like an idiot that can't account for latency times and give myself enough leeway.