When did we start caring more about the trees along the roads than about the people in the cars on the road?
And as I said above, it's a lot more than just the trees -- when you look at the impacts on aquatic ecosystems and drinking water reservoirs it becomes a bigger question of balancing the safety of people who demand that our taxes pay for road treatments that allow them to drive in icy conditions versus the public expense of removing salt contamination from drinking water, and the longer-term negative impacts on society from the degradation of natural ecosystems.
It's easy to get lulled into thinking that human systems operate independently from natural ones, but all you have to do is consider, for example, the impact of the crash in bee populations on agriculture to see that it's not true. Most fruits don't grow without pollinators (and by fruits I'm using the botanical term, so we're talking tomatoes, beans, peas, etc. as opposed to simply oranges and apples), and now farmers have to pay to have their plants pollinated because bee populations have been decimated. Those costs get passed on to consumers. I'm not drawing a connection between salting roads and dying bees, just illustrating that it's easy to say "why care about a few trees" when in fact the potential costs and ripple effects could be greater than that.