I think it's worth noting here that the ruling doesn't forbid states from drawing lines based on voting-eligible population, but rather SCOTUS declined to force states to do that. They didn't say drawing lines by voting-eligible population was constitutional, either. They neglected to address that issue entirely.
I could have misinterpreted this a bit, but I think it is clear that this ruling doesn't bar states from drawing districts like that.
It looks like you have that right:
"Because constitutional history, precedent, and practice reveal the infirmity of appellants’ claim, this Court need not resolve whether, as Texas now argues, States may draw districts to equalize voter-eligible population rather than total population."
http://www.supremecourt.gov/opinions/15pdf/14-940_ed9g.pdfThe Supreme Court, and other federal courts may only hear actual "cases". This particular case was a challenge to the Texas redistricting which was based on equal total population, not equal voter population. In order for the Supreme Court to decide on the constitutionality of voter-only apportionment, a State would have to actually adopt the scheme and draw maps under the plan, and a legal challenge would have to work its way through the Fed Courts. So voter-only apportionment may be flat out unconstitutional, or OK. But in this case, the Supreme Court unanimously believes that it is permissible for States to base maps on equal population. Thomas concurred on this, but called the "one person, one vote" test a stupid fiction that should be eliminated. Alito also concurred on the outcome, but took issue with certain history presented in the majority opinion.
