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.”
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.”