Well, I'm not so sure. You have a point that when recieving free energy the bot doesn't need to do anything, still I've seen them do lots of stuff than just sitting fixed on the background. Last time I started out with veggies and then separated them into heterotrophs and autotrophs, ran them in two instances connected with teleporters. The heterotrophs I ran with energy providers to feed them. I found the heterotrophs evolved to do even less then the autotrophs, totally energy conservative. Also they became more hardy against costs, and eventually slowly started to take over the autotroph sim, just waiting for the autotrophs to die of too high costs at night.
This time around I started with both heterotrophs and autotrophs in the same sim, only giving the autotrophs 1 energy per cycles, plus the night lasts for 32000 cycles. That means autotrophs can't afford to do much if they want to live, or they need to evolve better feeding strategy. All it takes here is a bot that evolves tie feeding or shooting in close proximity of other bots to get that extra energy, and it should be able to survive and reproduce better.