European expansion was served very well by the superior technology and armaments available to whites. African natives had no access to those technologies; in fact, most of the indigenous tribes had no concept that they even existed until the European armies appeared on the scene. It is typical of historical expansionism that the invading armies took prisoners and slaves, and used brute force to compel others to follow instead of fighting back. For example, when Marlow is entering the Outer Station, he sees a line of native workers/prisoners, with another native acting as supervisor:
He had a uniform jacket with one button off, and seeing a white man on the path, hoisted his weapon to his shoulder with alacrity. This was simple prudence, white men being so much alike at a distance that he could not tell who I might be.
(Conrad, Heart of Darkness, eNotes eText)
The supervisor reacts with devotion to Marlow, even though he has never met him, because he has been trained to expect violence if he does not follow orders. Most of the other natives are compelled by force, even the ones who attack Marlow's steamboat; it comes out later that they were ordered to by Kurtz, who commands fear because of his brutality. Since the natives had no way of fighting the superior weapons of the whites, they had little choice but to become slaves or die.
I don't think they were allowing themselves to be bullied, or that they felt they had to bear it. They were slaves - their captors outnumbered and outequipped them greatly, and they simply had no choice but to obey the Europeans.
Essentially, I don't think they had any other option.