Why do people so often invert the burden of proof?

I know, right ?

If someone says “Picking your nose will cause brain-cancer in 40 years.” Then they have the burden to proof that. Nobody has the burden to disprove that.

Absolutely, and if you'd asked for proof of their accusation you'd be correct in this instance.

They made the accusation that this is a step to make this age fields mandatory, and controlled by third-party age verification services, so they have the burden to proof that there is way to do that.

They did and you could ask them to make a case for that, you didn't.

You provided your own accusation:

You do know that this is a slippery slope argument, right?

And proceeded to tell them that they are required to provide proof to dispute your new accusation.

You would have to demonstrate that there is an intention there to require third party services to validate the age of users using Linux… Or that there is an intention to do so by systemd and the broader open source developers.

Which is what i was addressing specifically when i said:

You , as the party making the accusation of fallacy would be required to prove that the expectation of escalation is unreasonable or that the intention was not there.

I find it highly unlikely, because most people using Linux systems at home have admin privileges. Which makes this whole point moot, since they can fake whatever they like to the software running on top.

It makes the field itself mostly a non issue in the single isolated context of "does this field, on it's own, constitute age verification".

The point most people are trying to make is that it's a part of a larger context.