The extreme power of Talosian telepathy and the potential havoc they could wreak with it is far greater than that of even a rogue starship commander. Federation security demanded the Talosians remain in utter isolation to ensure that they could never be a threat.
The death penalty was extended to McCoy and Scott for discussing making a move against the Captain - their conversation in the corridor was recorded and played back in the hearing, for all to hear. McCoy and Scott were guilty of conspiracy to commit mutiny (albeit justified mutiny). And besides, the phrasing and delivery of the lines just doesn't make sense for them to be arguing that they weren't guilty; Sulu's not saying the death penalty's forbidden because Kirk hasn't made a case for it, he's arguing that the punishment doesn't fit the crime. Later, when he's talking to Chekov on the bridge, he's not protesting that McCoy and Scott are innocent; he's protesting the mere fact they're having an execution, and themselves decide because of this to ignore Kirk's orders from then on.Because they were probably convinced that Spock et al were innocent of mutiny.
It's not entirely important that it's four or seven - what matters is it's a general order that nobody's broken, and hence the death penalty is unjustified.Perhaps... except no-one corrected him. Sulu was following his exact train of thought, but didn't stop to say "Yo Pavel, it's GO7 - geddit right."
