It's very much not okay. In general, imposing labels on people is a bad thing. Doesn't matter how much better someone thinks they know, people get to explore their identities in their own way and at their own pace.
When I see someone imposing the egg or trans label I generally point them to the egg prime directive here.
When someone is just told they are trans, that opens ground for denial; it activates defense mechanisms built by internalized transphobia, and it has a high probability of pushing them further into the closet, if not making them outright transphobic. Even when it doesn’t, it leaves ground for their own subconscious to reject their dysphoria, claiming that they were just manipulated or deceived. The much more effective strategy is to talk about your own experiences with dysphoria so that they see the common grounds and come to their own conclusion about their gender. The code doesn’t forbid helping them to explore their gender; it forbids assigning a gender to them. Or, to put it more succinctly, you cannot be told what the Matrix is; you can only be shown.
As to why some people don't follow that. Either they don't know better, in which case educating them should fix it, or they are simply [epithet of your choice here]. Any community of sufficient size will have bad actors in it.
That's exactly why it needs to be something you are willing to explain. It makes their stories better.