Surely this boils down to some common sense, and while being rudely asked to leave for a brief visit is not the way to go, these piers are owned and built at great expense for the use of a resorts guests. They are situated on public land but they are someones property. The access law/rule seems aimed as an access for emergencies at sea and possibly for short term docking. You see hoards of people camped out for the day on some docks and it precludes the tourists the opportunity to use the facilities designed for them. That would annoy me as a resort owner that had spent $100k+ building one and thousands more every year maintaining and insuring it. There is some middle ground in my view but if people don't want you on their property, and they're prepared to take any poor public perception as the consequence, so be it. I see even Ramons, the most tolerant resort to non guests using their facilities, have even had to take the step of marking their sunbeds for use of guests only recently. It's the same issue but different property (and value)
An analogy is your car or golf cart. It's used and often parked on public roads, but do you let any Joe Bloggs drive it around when you've left it parked (although I'm sure we've all had it happen), leave a mess in it, and possibly damage it without telling you.