They are already doing it without warrants... Everything you say and do on a cellphone is being recorded. Not only that, it is being recorded and sold.
How is it legal? Because when you agree to use their device and service you are waiving all your privacy rights by contract. To use the device you have to legally agree that they can collect and use it all as they like.
Contract law prevails with this.
You may be right. But if the device that my friend is carrying records me, I have no contractual relationship with the device provider.