This isn't totally true, at least not for the casual use of "cause". Cops must have "reasonable suspicion"[1][2] to detain, during that detainment they can discover more information which creates "probable cause", and probable cause allows them to arrest.
My guess is they were arrested for what is commonly called "resisting arrest"[3]. The way resisting arrest laws are unfortunately written, obstructing an officer in their duty is enough, they don't have to actually be resisting arrest. Basically of the cops told the CNN crew to get off the street, and the crew didn't, that would be "resisting arrest".
Minnesota's law:
> obstructs, resists, or interferes with a peace officer while the officer is engaged in the performance of official duties
Note that they were arrested, you can hear an officer say they are under arrest, but they haven't been charged or convicted. I doubt they will be charged. It is up to the county district attorney to press charges (the same DA who isn't pressing charges against the officer who killed George Floyd).
The press in the US are usually given a lot of freedom, and are allowed to be in places that the police don't allow regular citizens (riots areas, wildfire areas, etc.).
[1] https://en.wikipedia.org/wiki/Reasonable_suspicion
[2] https://www.nolo.com/legal-encyclopedia/what-reasonable-susp...