I thing your interpretation of SPE is too static. If memory serves, these folks did not "jum" into their ascribed by the experimenter roles right away - there was some trepidation at the beginning. But th eunequal power relations developed quite quickly as a result in _interaction_ between these two groups. That interaction was defined by the following circumstances: (i) one group had the capacities that the other did not, (ii) they use of those capacities was sanctioned by culturally defined roles, i.e. legitimate, and (iii) there was a considerable absence of feedback from exercising that capacity, exacerbated by the fact that it was only an experiment (in that way, SPE was more like a B/D game in a bedroom).
The bottom line is that "power" asymmetries are not pre-scribed apriori by social roles and relations - they emerge from social interactions in which prescribes roles, expectations, and capacities have some, but not necessarily decisive, influence.
Wojtek