I’m not sure that it deliberately is, TBH. I understand your point re the Registration process but I don’t agree that labelling the button Ask would make it less ambiguous – I’d say the opposite. I think that most people would intuitively click Log In if they wanted to sign up.
I also think it is important to consider how people would usually use a Discourse site on mobile if they had never been to the site before. They would almost certainly have followed a link from a search engine or other site and would land on the topic they were trying to find out about. Once they had read it, they would either try to reply and be prompted to create an account or they would get the JIT message that Moin mentioned.
Alternatively, they may visit for the first time because they want support from the site, in which case they would search and browse and the same thing as above would occur.
Most people do not visit the homepage of a forum for the first time and decide to immediately sign up, so we have designed around those more common paths.