After operation cast lead in 2008, IDF declared the north tip of the Gaza strip, from which terrorists would routinely launch rockets at Israeli towns, a NO-GO ZONE. You've been to the army, you know what a no-go zone is. What do you do when repeated warnings not to enter the no-go zone go unheeded?
If you have authority over that land [..]
But you don't. It's in the Gaza strip.
You don't knowingly shoot at unarmed civilians unless you are a terrorist.
It takes less than 30 seconds for an "innocent farmer" to whip a mortar out of his truck and fire a shell at Israeli civilians across the nearby border, after which he immediately returns to being an "innocent farmer". That's why IDF declared the buffer zone - up to hundreds of projectiles were launched at Israel daily from those areas by "farmers". I'm sure that there was official warning given, I'm sure shots were fired in the air first, and it's reasonable to assume that on the first few days the soldiers went no further than that.
Perhaps you should ask why the farmers kept coming back to a declared buffer zone along the Israeli border in the midst and peak of a hostile confrontation (Cast Lead). And don't tell me "to farm their lands" - everybody knows that when there is a war you stay out of the battlefield. And why, as the video clearly shows, did they not run for cover when first shots were fired. And why a production team of "peace volunteers" just happened to be there with a camera and a megaphone to direct the action.
If Hamas declares Sderot a no-go zone, does that make it ok for them to target civilians there?
. To declare a no-go zone there has to be a military reason. It has to be a battlefield at the front line, it has to be a place where hostilities are carried out against you. What hostile threat does Sderot pose to Hamas forces or Gaza population? There are no bases there and no artillery positioned there. Hamas can't (by lawful standards) declare Sderot a no-go zone any more than IDF can declare Central Gaza a no-go zone.
(It also generally makes no sense to declare a no-go zone where people live, since you want civilians to stay away from the battlefield and in their houses. This however is a standard that was hard to implement in Cast Lead since Hamas would mix-n-match with civilians in densely populated areas.)