The difference there is that the basis for restrictions comes from already established precedent surrounding regulation of firearms. It would be illegal for the layperson to build, but not necessarily someone licensed to do so.

In this case, there is no precedent for requiring an ID to use computer hardware. The basis (as far as I understand it, but I'm not a lawyer) being used to force these ID laws is that these OSs (Google, Apple, and Microsoft) are actually a commercial service which knowingly deals with children, and there are established laws about how companies are allowed to enter into service with children and obligations/regulations that require a good faith attempt to determine what sort of customer they are dealing with.

In the case of FOSS, though, there is no associated company and no contract of service, and no reasonable way for a developer or maintainer to know who is using their service. It's much closer to someone giving away a book than it is to basically anything else, and there are decades of precedent backing this up, which makes it a much easier thing to challenge in court.