Sorry I think the level of snark is unnecessary here, its a valid question on developing creativity and imagination - snarking just because Mumsnet allows it is childish and silly.
I don't think you are unreasonable to question it and think about it.
You would be unreasonable to go storming in, let rip at the teacher, hoik child out of nursery and go to the daily wail...
But thats not what you were saying at all!
As someone who does artistic things professionally (well, semi.. ok, people pay me for my work!)...
Copying IS a valuable learning tool, absolutely..
But equally, having the freedom to express oneself (damn this is hard to write without sounding like a cosmic twat) without conforming to 'THE RULES' is also really important.
I think you need to encourage experimentation with both really - I know that feeling like I had to stick to 'the rules' caused me quite a lot of stress as a child (even as quite a young child).. the thought that I was 'doing it wrong' actually put me off quite a bit.
However, having come back to art as an adult, now I have the confidence to do what I want, I also find learning 'the rules' pretty useful.
The two things I think that fucked me over FAR more as a child however than being pushed to conform to the rules, were..
1/ Being given crappy 'tools'.. shite paints, horrible cheap nasty paper, rubbish pencils.
Someone with poor motorskills as any small child has in comparison to an adult, is not going to get BETTER using rubbish materials. I am not saying toddlers should be given colouring pencils that cost £50 a tin, but try materials out yourself and if YOU think they are shit, and hard to use, and frustrating because they don't do what you want them to do.. your child certainly will too!
2/ Lack of parental enthusiasm/appreciation.
I don't care how often your child draws something to a mediocre standard - act like its fucking awesome unless or until they ask you for help to improve. If THEY are happy with the job they did, thats good enough, the time will come when they recognise the difference between what they aimed to achieve and what they actually did and they will ask for help and constructive criticism when that occurs (you can prompt that a bit by carefully asking if THEY are happy with it of course!).