This makes sense: if a rule has a variable that can be filled by any English preposition, there is a risk that an incorrect preposition will fill it.
splitting at a period is a safe bet, and frees the model to use rules that dig deeper into NP and VP trees when constructing a top-level S. Table 5 shows weights for generated English nonterminals: SBAR-C nodes are rewarded and commas are punished.