It's more accurate to lob something than throw it like a clemmy in this instance, as there's a chance it'll not get very far throwing it the 'clemmy' way.

I'd have thought you have more chance of landing it on target the arc'ed way, gives it less chance to skim as it comes down near vertical

Most of the time they are lobbed from behind cover if you had to throw them like you want to, you would probably be shot.
But what do I know I was in the R.E.M.E. the only thing we threw were strops when the bar didn't open

Both reasons are covered above; throwing from cover and accuracy. That said, in real life I never seen one thrown as per the training manual.