Prior to the invasion, Baron Harkonnen says:
There are no satellites over Arrakis. The Atreides will die in the dark.
In the novel, we're given to understand that the Fremen bribe the Spacing Guild with spice, so that the Guild will keep satellites prohibitively expensive.
However the noble houses all have their own spacefleets. It should be trivial for them to launch a few satellites of their own into orbit. What prevents them from doing so?
The Spacing Guild forbade it.
Halleck said: “Wouldn’t it be cheaper to reopen negotiations with the Guild for permission to orbit a frigate as a weather satellite?”
The Duke looked at Hawat. “Nothing new there, eh, Thufir?”
“We must pursue other avenues for now,” Hawat said. “The Guild agent wasn’t really negotiating with us. He was merely making it plain–one Mentat to another–that the price was out of our reach and would remain so no matter how long a reach we develop. Our task is to find out why before we approach him again.”
Dune
Frigates and ships presumably come under the general heading of "and such", that the Fremen pay the guild to keep out of their skies.
Jessica stopped in the act of turning away from him, looked back up into his face. “The Guild? What has the Guild to do with your spice?”
“It’s Liet’s command,” Stilgar said. “We know the reason, but the taste of it sours us. We bribe the Guild with a monstrous payment in spice to keep our skies clear of satellites and such that none may spy what we do to the face of Arrakis.”
As to why Leto Sr. doesn't just ignore this edict, note how paranoid he is about anything that might impede his ability to contract with the Guild.
“I’m going to watch our screens and try to see a Guildsman.”
“You won’t. Not even their agents ever see a Guildsman. The Guild’s as jealous of its privacy as it is of its monopoly. Don’t do anything to endanger our shipping privileges, Paul.”
Unauthorised deployment of satellites would result in a Guild embargo, resulting in loss of trade and communications with the rest of the Imperium and resultant poverty, isolation and loss of influence - ultimately resulting in loss of the fief. "The Spice Must Flow"
