So why can't they say it plainly and forceably?
The US Supreme Court does not grant new civil rights as cases appear. They protect civil rights which in turn are protected by the Bill Of Rights, Constitution and other documents and rulings.
The Bill Of Rights took effect in 1792. No reasonable government official of 1793 would assume that since the US Supreme Court had not yet made rulings related to the Bill Of Rights in any detail, torturing a confession out of somebody would be constitutional.
That is exactly what the Moore court has ruled: since the US Supreme Court has not yet ruled on "bearing arms", no constitutional right to "bear arms" exists, even though any possible reading shows the same group of people must possess both the right to "keep" and "bear" arms and no scholarly authority has ever tried to differenciate the two.
How simple is that?