It might be the way you're trying to utilize them.
If you click on an item in your inventory you want to throw, you "Equip" it, which costs a standard action. You then throw as a standard action.
However, if you use the "Throw" action, then select the item you wish to throw (From that messy UI bar that will show your entire inventory) then you can throw the object with just a single standard action.
Took me a little while to figure this one out too, since it was not particularly intuitive.