If we want to take BC as canon, then we have the Turks in a position where one false step will lead to their deaths. To me, though, this would be a reason to say, "screw these orders, I'm outta here", and flee into hiding like the rest of their compadres. However, they can't do this because Rufus needs their support
If the game had wanted the player to think along these lines about the Turks and their involvement in the platedrop, they had the opportunity to put this information in the script. They didn't. But who knows, maybe they're saving it for Part II.
Throughout BC, the Turks do increasingly defy Shinra's orders, though never on a whim, always when some major personal issue is at stake: saving Zack, saving Veld. Also, the player BC Turks question Shinra much more than the NPC Turks - Rude, Reno, Tseng.
Rude, Reno and Tseng are accustomed to following orders rather than listening to their consciences. They have a habit or culture of obedience, which they take enormous pride in. Their consciences were probably not very loud to begin with; nobody can succeed in their line of work if they have a conscience that is constantly intefering with their orders.
I'm trying to think of other instances where they point blank refused to follow orders.
- The Wutai sidequest. Two grunts run in and expect the Turks to drop what they're doing and chase Corneo. Reno doesn't understand the rush. They'll catch Corneo in their own time. This isn't really refusing orders so much as refusing to be ordered around by a couple of troopers much lower down on the command totem pole. And they do indeed deal effectively with Corneo, in their own time, in their own way.
- the Wutai sidequest, refusing to capture Avalanche. Again, Reno hasn't refused the order. He has a kind of rogue's honour that won't allow him to turn on people he was cooperating with five minute earlier, so he's giving them a head start. If they're still around in Wutai tomorrow, he'll deal with them.
- in the Midgar tunnels. The end of the world is hours away. Fighting Avalanche seems pretty pointless now, but they'll do it if you insist
I'd say it's less a case of point-blank refusing to obey orders, as of demanding and expecting to be allowed to use their own judgement as to how those orders are carried out. They are obedient, but not blindly so.
Why wasn't the plate-drop the line the Turks could not cross, the order they found they couldn't bring themselves to carry out? It's an interesting question, by which I mean it's a question that helps to make them interesting characters. They've been doing bad stuff for so long that the difference between the platedrop and all the previous things they've done in the line of duty is merely quantitive, not qualitative.
And I've said it before, but I'll say it again: I think the game writers did a great job of showing their doubt and hesitation, and then turning the struggle on the pillar into a personal battle between Cloud, Tifa and Barret on one side and Rude and Reno on the other. The Turks are no longer trying to drop a plate on 100,000 people's heads. They're trying to defeat Avalanche. They're no longer besmirching their honour by participating in an atrocity; in the heat of the moment, they are defending their honour by refusing to let the enemy get in their way.
I imagine they are both practiced and trained in not dwelling on thoughts of guilt after the event. Nevertheless, they can't shake it off this time.
I have no doubt that in the Remake the writing will make it clear that the guilt they feel affects all of their subsequent actions, whether those actions aim to justify, or atone for, their involvement in the plate drop.
