Bad answer, pschearer! if you are a brain in a beaker, then all your sensations - including pain and pleasure - will be the result of an electrical stimulation by this computer. It doesn’t work either for solipsists, because solipsism holds that the external world and other beings are creation of one’s own mind: this kick in the shin would just be part of your imagination, just like in a dream you are afraid of monsters your imagination created, or you feel pain or pleasure from stimuli your brain created (see erotic dreams for example).
This comic is philosophically very interesting and very logical! you will never be able to prove that you are not a brain in a beaker, because you will never be able to prove that this idea that you are a brain in a beaker doesn’t come from your mind!
Yes, competition WOULD keep them in line if they didn’t rig by passing secret contracts and eliminating it!