It's nothing but a marketing trick to draw more sales if you ask me. The terminology came into use via Young Living EOs- a multi-level marketing company that was recently sued by the US government for making healing claims in their advertising. Krista is correct, though- there really is no such thing as 'therapeutic grade' in the EO trade.
For me I stopped using Bulk. I live in NorCal and the shipping was outrages. I found a supplier that has better quality eo but they are pricey. I prefer to pay more for quality and less for shipping. Works out to about the same.
I dont care for the eo that Bulk carries. Iam very sensitive to fragrances; I had ordered 16 oz of Lavender eo......for like noting in price; when I took one whiff, it just about knocked me over. I called Bulk and they said that it was some type that they mix with God only knows what.