I think the principle of allowing children to choose without forcing preconceptions on them is the idea behind it - albeit a short sighted one.
The issue is primarily that because of this 'counter culture' so to speak, the norm (as in that which already exists) is demonised to try and pave the way for the 'new culture' but in doing creates stupid partisanship such as statements like seeing a girl in a dress 'enforces gender norms and is oppressive' or literally the 'you're either with us or our enemy' five year old clique logic that circulates among those at the further ends of the spectrum ie extreme progressives/conservatives.
It doesn't recognize firstly, girls and boys do have differences in brain structure (indeed this is the underpinning for the legitimacy of transgender as a genuine deep personal issue) which is observable even from birth by the toys they pick and innate behaviour. This leads to a 'gender norm' as that brain pattern behaves in a certain way. There is no issue with this. If a child does not associate with the item in question they'll let you know - a 'tomboy' child of a family friend would scream at the thought of being put in a dress basically from birth, it just didn't 'feel right' to them and this certainly wasn't an opinion enforced by society as tomboys are not particularly prevalent. People cannot help their inherent brain structure and it does give them certain dispositions for example women are more happy to be at home due to the evolutionary instinct, and men are more aggressive and have a higher percentage of muscle due to same. Men and women store fat differently for evolutionary reasons - to ignore the fundamentals that make up our species is mad.
It's important to allow people to be outside these norms, but forcing them even as a way of 'counter culture' is not the solution. For example if - without prompting - my theoretical son wanted a barbie I'd allow them to have it. I'd let them bring it home and then warn them of the potential social consequences for their own wellbeing ie don't be ashamed but for an easy life enjoy it more privately unless you want to defend it against other people's views in which case I'll help you do that. It wouldn't bother me he wanted a barbie - it's just a long haired action man. Kids go through stages in finding out who they are - better to let them unless they pose risk to themselves or others by doing so. Forcing them to behave, or not to behave, in a certain way just produces ill adjusted kids when it's against their very identity. It's not the same as stopping them breaking rules, that's boundaries, telling them they can't be who they are is awful.
I have enough faith that if a child sees a male doctor on TV they won't then say 'I'm female so I can't be a doctor', especially not when they're older. My go to example is if men are represented as fat, dumb and lazy on TV ie homer simpson, peter griffin, nobody watching that says 'well I'd better not bother trying as hard as I can in my exams because I'm clearly dumb as well'. That doesn't make sense on any level. Even if they do think they're dumb they'll still try hard on their exams and life experience will give them their opinion ie if they were straight A's since birth then they won't think they're dumb. If they can't hack education, they'll think they're dumb - or more likely blame the system and become wayward because nobody likes being told they're stupid. Point being this whole 'I saw a thing once on TV and it told me who I have to be' is nonsense. Most of TV is designed to cater TO its audience ie they find out what people want to see and put it in there which is why lynx advertises as 'unlock your inner greatness and get all these chicks' whereas Maybelline advertises as 'you're worth it'. They appeal to the thought pattern of their demographic, they don't define it.