Stealing requires that something belonging to one be taken illicitly. One who watches a copyrighted work (leaving questions of material cooperation aside) doesn’t take anything away from the copyright holder. That person is in the same position as he would have been had the viewer done nothing.
For the same reason, the operators of illicit streams are not guilty of theft, though they are guilty of unjust damage. This distinction is practically relevant to the question of restitution, since the money made from an illicit stream will not generally be equal to the damage caused (it may be more or less).