That is sad yet funny at the same time. Sorta reminds me of Gore's "I invented the internet" comment.
Disney in no way owns the image of Santa Claus. If this poor chap is walking around the park trying to enjoy himself, and kids keep approaching thinking he's Santa, he's kind-hearted for stopping and playing along. But if he came dressed as Santa for the sole purpose of drawing crowds for autographs, etc, then of course Disney has grounds to ask him to stop.
I know an individual who resembles Santa very highly, even in street clothes. He's a mall Santa in Nov/Dec, but his white hair & beard do not go away the rest of the year...so naturally he fits the part. Even in street clothes he's stopped quite frequently by families w/ small children. And being the type of person he is, he's not going to brush them off: "Go away kid...I'm not Santa Claus!".
There's a difference between playing along for children's sake, and intentionally forming a mini "meet & greet" session. I highly doubt this Santa was doing the latter.