I wasn't going to jump into this as originally it was about horse racing and my own interest is motorsport, and a specific branch of it, but as someone else has raised the subject I'll dive in with my 2 cents ...
I attend many short oval motorsport meetings, stadiums such as Foxhall near Ipswich in Suffolk where I live, mostly to support family and friends who compete in various formula at these events. I take pictures and share them on social media, and I know a few other guys who do the same, one who takes a LOT more photos than me and is very good at what he does (I'd say he's a far better photographer than the "official" togs, I wouldn't say I am).
At all these events there are official togs, who take a load of photos and attempt to sell them from their websites, and one in particular who sells photos from a small shop on-site. My understanding is some, though not all, official togs see us "other side of the fence" amateurs as a risk to their business, for the reasons described in this thread, and I can't say I disagree with them, but the question remains, does the sport exist to provide the tog with a living, and if the tog went out of business because he / she couldn't compete with the amateur who gives away his efforts for free, who suffers, other than the tog?
One could argue the official tog, due to having access the amateur doesn't, should be able to take "better" pictures, and as such should still be able to sell them. At the events I attend the official togs take poor pictures which I'd be surprised anyone would want to buy, but by contrast I went to Cadwell Park not long ago, and the official tog there not only shares (with unobtrusive watermark) 100's of very good images on social media, but partly by virtue of access he has that we don't, and mostly by virtue of sheer talent, he produces such stunning images he can still sell enough to make his business viable.
So one answer to behind the fence amateurs giving away stuff for free is to ban them (or their gear) from the venue so they can't, but I'd argue a better answer is for the track-side togs to raise their game and produce images people are going to want to pay for, and if they can't go do something else, because the sport won't suffer their loss if all they do is take snapshots anyone else could take.