This is just a standard part of Directive 2004/38/EC. Same provision is in Irish law. And same provision in Schengen.
Is my conclusion valid? Ie, when only visiting the UK, it is thus NOT illegal to "just" cross the border from Ireland, as long as a visa-national is married to an EU-citizen?
Another question, maybe not that relevant, is: The Isle Of Man sais that it's "legal" to enter the IOM, when it's legal to enter the UK.
This leads to difficult questions: If the UK does (illegally) not implement an EU-law, and the IOM sais "we do as the UK does", then I guess the UK can be sued by the EU, while the IOM can not, as it's not in the EU.
So if the UK (illegally) sais "you need an EEA-family-permit", then that's actually illegal by the UK, but legal in the IOM?
(I'd actually really like to know, as due to a lack of alternatives it's be nice to go there for a weekend

)
---> The EU should agree (even without an EU-wide Schengen) on ONE card or visa, that allows travelling EU-wide. This would allow border-guards to most easily identify who may travel, and who may not.
---> The current chaos serves no-one, and just cannot be very secure. If they can't even determine who may travel, I actually do not trust them to handle a database of "unwanted" people, and actually use it...