While Frontier charges for "carry-on" bags and requires that your "personal item" fit under the seat, as far as I know there is no requirement that you place you place your "personal item" under the seat. In fact since Frontier charges more for a "carry-on" than for a checked bag and since many people are perfectly fine with traveling with a back pack as their "personal item" the overhead bins are usually almost empty. I've never seen an FA tell anyone that they could not put their "personal Item" in the overhead bin.