Do most flight schools and FBOs in the US prohibit student pilots from practicing stalls while flying solo?
The two flight schools I have used had this rule and all of the other local places I know of also have this rule. The FAA no longer recommends full break stalls during training anyway you merely need to bring it to the horn. Interestingly enough the data does not support the decision 


  Student pilots are, by far, the least likely to suffer stall/spin
  accidents, as a proportion of in the pilot population. Pilots holding
  FAA Airline Transport Pilot (ATP) certificates are also less likely to
  stall/spin.


Although a case can likely be made that limits like this are what make student pilots least likely to stall/spin.