One might think that electronic surveillance, using the so-called "stingray" device mentioned in the article, is replacing old-fashioned let's-go-follow-him surveillance of the type you might have seen 40 years ago on, say, Hawaii Five-O or Barnaby Jones. In that case, there would seem not to be much controversy. However, as cell phones have become such standard personal accessories that they may be considered virtually an appendage of a person, any tracking of cell phones or similar devices can be considered a "search" and the monitoring of them can be considered sufficiently intrusive as to implicate concerns -- or objections -- based on the Fourth Amendment prohibition against unreasonable searches and seizures. (Note the emphasis.)
As the article notes, there are pending bills before Congress to require that judges issue warrants for the use of the stingray mechanism. The Congressional action is an appropriate check on the executive power (used through law enforcement agencies).
Eric Dixon is a New York lawyer, strategic analyst and crisis management consultant who helps people cope with the stress of litigation and being under investigation.