The Autobots didn't see Megatron suddenly appearing out of nowhere heading for Optimus & Sentinel on the bridge? I mean they could have thought that Megatron was about to join in the fight with Sentinel against Optimus, they might never know. Well, the rest of Autobots might be busy fighting other Decepticons or tackling pillars or shouting to one another to GET DAH PILLARZ DOWN, but a huge rusty truck with a shotgun in the hand is rather prominent in a battle zone right? But anyway, when the pillars were down, Megatron was proposing a truce to Optimus, what were the rest of the Autobots doing? o_o Taking their own sweet time and stroll to the bridge or standing at one end of the bridge and watch Optimus throw an axe into Megatron's face and then shoot their former leader in the face twice?
Well, if they did see him running up to Sentinel and Optimus, they were probably hoping for the best. Sentinel would have killed Optimus anyway.
They said "Quick, we need to distract Megatron and lure him away from Optimus!" and then they drove away.
They knew better than to intervene, or Optimus might have taken their faces, too, in his state of battle frenzy.
My catch all answer for why any character wasn't there is they stopped to get tacos. So the other Autobots would have gone to help at the bridge but damn that taco truck.
The Autobots were most likely still fighting the fighters that Sentinel called. Megatron was sitting in an alley so they probably never even saw him pop out until he actually started firing at Sentinel. It's not like there wasn't plenty to distract them.
Sideswipe: "Autobots, give Optimus some backup!" Dino: "But, what if we just get in Optimus' way and he takes our faces?" Ratchet: "Yeah, that'd be awful. He might just rip through us as if we were Decepticons." Sideswipe: "Huh. You're right, it's a suicide mission. ... Hot Rod! Go help Optimus!" Hot Rod: "You got it! I'm coming, Prime!"
Since the autobots can zip through the streets as "robots in in disguise" without the drones finding out, give megatron a chance for him to pull off the same thing, misleading the good guys that he's a generic NEST military truck heading towards Prime.
What is this "drone" you speak of? Perhaps you mean the unarmored Protoform Decepticons in the city of Chicago?
What if it was like this episode of Firefly I watched. Crew approaches Mal with guns while he's fighting some guy. Zoe: Wait this is the captain's battle. Mal: No it's not! The crew shoots the villain dead.
