until he can draw a person as 2 circles with eyes and mouth and put arms on the body (not the head) then he cannot conceptually differentiate between an 'a' and 'b', and an 'n' and an 'h' and so they are not ready to write.
My DS (4 1/4) has quite poor fine motor skills, especially around writing/drawing, and cannot draw a person that looks like this. However he knows all his letters by sight and can point out to you where an 'n' differs from an 'h' and is starting to tell you how to distinguish 'p' from 'b' (which is more advanced than 'a' vs 'b')
So while we are still trying to get something other than a fist grip, using special crayons for scribbling drawing, working on threading, manipulating things, he is well able to begin some phonics, is good at rhyming and initial sounds, and can even identify the beginning, middle AND end sounds of some simple words. He even knows one digraph ('oo').
I don't think this can be a recognised developmental check that you MUST pass before writing or working with letters, and some children will not be that bothered by drawing people but really keen on writing. I know that generally, children who have more advanced skills will draw more advanced people but it's not necessary to do this before working on letters.
But some of the children in his nursery room can write their names without help, and copy letters well enough to make recognisable signs for things around the room.
However I bet some of them love drawing but couldn't care less about writing so the OP INBU in saying that they should not be forced to practice writing in this specific way. Other motor skills practice, and other pencil skills practice, will be more fun and just as valuable.
Incidentally if you tried to get my DS to do this he'd trace part of his first initial, scribble over the rest and run off. He would, however, sound out some of the letters if asked.