On what grounds where they arrested?

Edit: None whatsoever.

… after warning them they would be “subject to arrest,” for remaining on “Amazon property,” amid a crowd of protesters.

The Teamsters say the incident took place on public land outside the warehouse, where police have informed workers they can safely protest.

aKshuALly:

An NYPD spokesperson said the agency did not have an arrest record on file for the incident. They said a criminal court summons did not qualify as an arrest.

So there was no arrest at all. They just took them in for their own good ya know :)

Also a cool line from this washington post article:

Amazon founder Jeff Bezos owns The Washington Post.