Why isn't the president required to use a secure phone?

Until now, I thought that the President of the United States was REQUIRED to use a secure phone. However, if you do a Google or Bing search asking about the topic, all you find are headlines about the steps that President Obama had to take to get a secure phone. I remember reading about all of the steps as they happened, and I was under the impression that Obama had no choice in the matter. Now, however, it seems that Obama was simply following the advice of his security team, and the new president is under no obligation to use a safe phone.

For those who don't remember what a big deal it was for a president to have a smart phone, here is some background. After he was elected, Obama turned in his Blackberry, and carried no mobile phone until he was given a new, extremely restricted Blackberry. When he wanted an iPhone, he again had to wait for a secure, limited version. His iPhone allowed him to surf the web and correspond by email with a small group of people, but it had no camera, music, or text. He used the White House's internal switchboard to speak with government officials, and he had a competely separate phone for tweeting, not that he tweeted much. (A good summary of all of this is in this recent article.)

Today's headlines say that Trump "might possibly" be using a secure phone, although no one is quite sure what he's doing. (I guess this is something his staff don't feel safe lying about, because they could claim he has a safe phone, even though he doesn't. Either they want the public to think that mobile security is not a big deal, or they think they would be caught in that lie.)

It seems like the government cybersecurity community has been assuming -- with good reason -- that all presidents would behave like Obama when it came to following their recommendations. Now that we have seen that presidents don't always take cyber security seriously, maybe it's time for Congress to put a few requirements in place. There are simply some things that a president should not be allowed to do, and one of them is to use an unsecured phone.

I would love feedback on this topic, especially when it comes to any obligations already on the books.