This is an interesting article, but the headline is deceptive. The article doesn't answer that question it poses. The reason the Supreme Court granted cert is because the court below, which in this case was a special 3 judge district court panel whose opinions skip the Court of Appeals, ruled in favor of the challengers. If the Supreme Court had not granted cert it would have meant that this lower court opinion would…
That's the procedural reason -- the case ended up in the SC because the lower court sent it up the chain, but that's still not an explanation for why they chose to accept it. As far as I know, they still had the option to refuse it.
See: http://electionlawblog.org/?p=91947 for more info.