In 2008, the Supreme Court decided Crawford vs. Marion County Election Board. Previously, the state of Indiana had passed a statute requiring voters to show photo ID at polling places, something that was likely to disproportionately hurt black turnout. Indiana's justification for the law was its interest in preventing voter fraud, something that they were unable to demonstrate even a single case of. Nonetheless, the court upheld the law under this reasoning:
If a nondiscriminatory law is supported by valid neutral justifications, those justifications should not be disregarded simply because partisan interests may have provided one motivation for the votes of individual legislators. The state interests identified as justifications for SEA 483 are both neutral and sufficiently strong to require us to reject petitioners’ facial attack on the statute. The application of the statute to the vast majority of Indiana voters is amply justified by the valid interest in protecting “the integrity and reliability of the electoral process.”
Today, the Supreme Court decided Shelby County vs. Holder, an attack on the "preclearance" requirement of the Voting Rights Act. In 2006, Congress renewed the Act for 25 years, and after considering voluminous evidence decided not to make changes to the formula for deciding which states require preclearance for changes to their voting regulations and which ones don't. Nonetheless, the court overturned the law:
Congress did not use the record it compiled to shape a coverage formula grounded in current conditions. It instead reenacted a formula based on 40-year-old facts having no logical relation to the present day. The dissent relies on “second-generation barriers,” which are not impediments to the casting of ballots, but rather electoral arrangements that affect the weight of minority votes. That does not cure the problem. Viewing the preclearance requirements as targeting such efforts simply highlights the irrationality of continued reliance on the §4 coverage formula, which is based on voting tests and access to the ballot, not vote dilution. We cannot pretend that we are reviewing an updated statute, or try our hand at updating the statute ourselves, based on the new record compiled by Congress.
Note the difference. In Crawford, where the target is a law that's likely to disenfranchise black voters, the bar for constitutionality is almost absurdly low. Regardless of what the real motives of the lawmakers are, or what the likely effect of the law is, it's valid if the state merely asserts a "neutral justification." That's it.
But in Shelby County, where the target is a law designed to protect black voters, the bar for constitutionality is suddenly much higher. Even though the Fifteenth Amendment gives Congress the unconditional right to enact legislation designed to prevent states from abridging the right to vote "on account of race [or] color," the court ruled that, in fact, Congress is quite fettered after all. It cannot decide to simply renew a law that it thinks is working well. Instead, it's required by the court to update its formulas to satisfy the court's notions of what's logical and what isn't.
So here's your nickel summary. If a law is passed on a party-line vote, has no justification in the historical record, and is highly likely to harm black voting, that's OK as long as the legislature in question can whomp up some kind of neutral-sounding justification. Judicial restraint is the order of the day. But if a law is passed by unanimous vote, is based on a power given to Congress with no strings attached, and is likely to protect black voting, that's prohibited unless the Supreme Court can be persuaded that Congress's approach is one they approve of. Judicial restraint is out the window. Welcome to the 21st century.