I've come to this late and can see where both sides are coming from. However to just put the vendors point of view.

Walk a mile in my shoes...

Sometime in advance of any festival you make an application for a space and send a rather large cheque (which helps fund the event). You have almost no control over where you are pitched and if it's up a blind alley that's your problem and loss. It is possible you won't get a pitch at all (and may not be told until the last minute) in which case you either have to hope there's somewhere else to do or not earn anything that weekend.

The weather is out of your control, if it's bad you can't just stay at home, you've already paid. If it's part of a tour you may even already be there. So some hours before most people are up (or even the day before) you set up your stall and stock it. At some events you may even have to take down at the end of each day and put up again each morning, eighteen hour days are not uncommon. At some events you have to sleep with the stall for security.

At this point, just like the street performer, you are casting bread upon the waters, maybe you'll sell maybe not. If the organisers have had problems letting space you may find that half the vendors are selling the same sort of line (thank God I'm not a jeweller). Some you win, some you lose. The best hope is that over the season you make a living, many don't.

I would point out that vendors cannot just turn up and trade (even without a cover) if they did they too would be escorted away pretty sharpish (and might have their stock confiscated). A case in point is Sidmouth where there has been a crackdown on vendors trying to use a perceived (but actually non-existant) loophole to trade on the promenade. Even to trade (without covers) on a city street in the UK requires a Street Trading License from the Local Authority, in most places they're like rocking horse droppings.

So that's part of the walk in the vendors' shoes.

The big difference that I see is that the vendors are contributing financially to the running of the event as well as to the public's the experience of it. The street performers are contributing to the experience of it but not contributing financially even though the best of them may well be doing as well as many of the vendors.

As a Brit I have a number of issues with the interpretation of the US Constitution (a recent ruling that making and distributing videos of animal cruelty was protected under Free Speech provisions being one) so I probably attach less importance to this aspect of the argument than I should.