Well... maybe.
I grew up in a farming family, everything from wheat, grapes, tomatoes, potatoes, to walnuts and peaches.
All the farmers I know used to leave the outer two rows for people to glean.
Maybe that's because most of the farmers I knew went through the Great Depression and remembered being down on your luck.
But it also has to do with tithing for some.
I'm not saying every farmer does this, but a lot of the old-timers do.
My granddad would even plow up the outer two rows of potatoes and leave them for local people to come and pick.
I've been with him many times when he'd stop the pickup truck, get out, and help himself to somebody else's grapes.
He'd say, "Awww, he don't care - he expects it!"
So, I just don't know about the farmers in Spain.
Personally, I wouldn't pick fruit unless I asked.
But my experience has been when I have asked, the answer is always "Help yourself!"
Also, like Kanga said, many times those grapes you see on the side of the road are from thinning.
I imagine, each farmer is different.
Many of those grapes along the Camino are sprayed with an anti-fungal. It's bright pink, and probably not very healthy.
They used to spray cotton seed in the USA with the same thing.
My friends and I would play in the cotton trailers, and come out all pink.
We also used to dance under the spray of the crop dusters.
Now I have Multiple Chemical Sensitivities because of the chemical overload ... gee, I wonder why?