But if you call the company of course they'll tell you it's no good because then you'll go and buy more.
I think it's also a liability issue for them. If you were to call and ask if it were okay, and they said yes, and you used it and got sick, you might sue them. So the customer reps are going to err on the side of caution and tell you to throw it away, even if it's still perfectly alright to use. The layman's phrase for this is the "CYA response."
If you are worried that it might not rise enough, try mixing it up and making one cupcake with it. If it rises all right, then it's okay. If not, you might want to add some baking powder.