Retriever Towing Can this be legal? Portland, Oregon

We had never been to this part of Portland before.  We started to park in a lot, but it was clearly signed for use by patrons of Imago Theater and Jupiter Hotel.  As we were neither, we parked in a limited time space on the street, and looked around for a parking space that would accommodate the  duration of our expected visit.  I noticed a lot at a produce store, which was closed,  further down Ankeny.  I walked around the lot to check for "No Parking" signs.  I saw none.        We entered the lot from Ankeny.  This entrance was not posted "Exit Only"; however, after our vehicle was towed, a cruising Retriever tow truck driver did point out an arrow on the pavement....it was a small curved arrow white arrow that I had not noticed previously, and that even after it was pointed out to me, appeared to indicated left turn, rather than exit only.      There were no signs visible from any of the parking spaces, only a sign approximately three feet off the ground, hidden by bushes from the Ankney entrance facing the cross street.     I have subsequently discovered this lack of signage was not an omission, but rather their established business practice.  This is not the only lot where this condition exists.  The drivers do not respond to calls, but rather drive around and tow cars on commission.    I believe that the term Predatory Towing is a true and valid description of this practice.