For one thing, Reyes couldn't KNOW that either the Shedai or the Klingons would attack the colonists. Should French and British authorities not have allowed their people to establish colonies in the Americas because they feared the Spanish or native populations would kill them?
Reyes knew there were hostile forces in the area and if the colonists had not refused UFP protectorite status, he would have made sure soldiers and starships were present to protect them. But Reyes couldn't force them to stay off that planet, nor could he force Starfleet protection upon them. If the colonists would have allowed a Starfleet presence, would Reyes have sent soldiers to a likely death to protect civilians? You bet. He would probably have gone to his own certain death to protect his ex if she had allowed him to do so. Had he knew ahead of time the Shedai would in fact attack the colonists, he probably would have broken his oath before they left as opposed to after the massacre.
Saying he used them as human shields means Reyes intentionally put the colonists in harm's way to protect his secret but he wasn't sure of any immanent danger, only a possible/likely threat. He honestly had no idea what would happen. You can't play Monday morning quarterback and judge him after the fact, you have to judge him based upon the information and options he had at hand at the time the colonists disembarked.
Reyes didn't sent the civilians to their death because he was ordered to. He allowed them to expose themselves to danger because THEY refused to both accept a non-specific warning and accept military protection.
The French or British would not have hidden the fact that there were potentially hostile forces.
Imagine that you're imploding one of the hotels in Vegas. Let's say, in this case, the charges are on a random timer. They could go off any time in the next week. Do you keep your doors open and simply post a non-specific sign at the door? Or do you tell people that the building is wired with explosives and that there's a good chance that they will go off while they are inside?
And let's not forget that Starfleet was already on the planet. They were assisting the construction of the colony. And let's not forget that they had established a secret base within the colony. If Jeanne Vinueza knew that Starfleet had established a covert base on an independent planet she could have had all of the Federation personnel arrested as spies.
Starfleet was not simply checking to see if there was Shedai presence, they strongly believed that there was.
Reyes, and by extension Starfleet and the Federation, were willing to sacrifice the lives of civilians in order to keep the existence of the Shedai a secret. If Reyes' ex-wife had not been a member of the colony would he have even attempted the minimal warnings that he gave her? Let's not forget that she was an esper. She knew that he was hiding something. She knew that he was opposed to their refusing protectorate status. For all she knew he was lying to her just to keep them from becoming independent.
Lies built upon lies and paid for with the blood of innocents.
This is the same Starfleet that ordered the capture and, essentially, torture of the Shedai. That removed civilian oversight of the Shedai project on Vanguard. Starfleet, as the de facto military wether it's called that or not, exists to serve the Federation, a civilian government. Not the other way around.
And let's not forget how Starfleet also violated the Orion ship, killed it's crew members and later destroyed the ship itself. The same ship that was operating under the rules set forth by Reyes under the power of Starfleet.
And for all that, Nogura got promoted, presumably to CinC. These are not nice people.