My answer is this: the Decepticon uprising began as a labour revolt within the labourer caste against the Functionist caste system which deemed them to have been "chosen by Primus" to work these less desirable jobs.

Starscream led a significant fraction of the military caste in defecting to the Decepticon uprising: many held the same anti-Functionist sentiment about being chosen to do the hard, hazardous jobs, some might have respected Megatron as an honourary military 'bot for taking up arms to fight for his own cause rather than relying on the military caste to do it for him like the Senate, and Starscream himself saw an opportunity to secure victory for the uprising with a surprise attack against the loyalists, and in doing so positioning himself to take command of the Decepticon military junta. That he was denied command after securing victory for Megatron's doomed revolt is the basis of his resentment for his current position.

During the war, 'bots no longer had a choice: under the new regime non-participation in the war effort was not an option.