It's not legal. It just likely hasn't been explicitly made illegal. I'm sure every state has a law to operate motor vehicles with "due care" or some similar clause. Inattentive driving is not exercising due care.
I don't think the law matters here...it's enforcement. I cycle every day downtown in a major city, and I see so much craziness with phones, including watching videos, texting, and playing Pokemon Go while driving. My city has banned all use of cell phones while driving if not using a hands-free devices. It's just that nobody is ever pulled over, nobody's ever even warned, and it seems that nobody cares. The cops too are too busy on their phones.
By that logic, murder is "perfectly legal", as the title of the article suggests, so long as it isn't enforced. This is obviously a preposterous notion. If enforcement is the issue, the solution should focus on that. An article suggesting it's legal, absolves police of their duty to enforce laws and puts it in the hands of legislatures. But it's already illegal so law makers could do no more on the matter. Further, as the issue is enforcement, even if they passed more laws, it would beg the question how they will be enforced.