Lesson No. 1 here, of course, is that the reporters don't write the headlines.
Lesson No. 2: This is a matter of style, and Washington Post style calls for Roberts's. Styles vary, but if one style must be declared more correct than the other, Roberts's wins.
"The Elements of Style," in fact, makes the principle Rule 1 in Chapter 1. Sayeth Strunkwhite:
1. Form the possessive singular of nouns by adding 's.
Follow this rule whatever the final consonant. Thus write,
Charles's friendExceptions are the possessives of ancient proper names ending -es and -is, the possessive Jesus', and such forms as for conscience' sake, for righteousness' sake.
the witch's malice
The aggrieved readers' aggrievedness is an interesting demonstration of how well Americans have been "trained" by Associated Press style. It is true that most U.S. newspapers, because most U.S. newspapers follow AP style, would write Roberts'. AP says:
SINGULAR PROPER NAMES ENDING IN S: Use only an apostrophe: Achilles' heel, Agnes' book, Ceres' rites, Descartes' theories, Dickens' novels, Euripides' dramas, Hercules' labors, Jesus' life, Jules' seat, Kansas' schools, Moses' law, Socrates' life, Tennessee Williams' plays, Xerxes' armies. (An exception is St. James's Palace.)
There are myriad rules within the rule -- tiny points on which virtually no two stylebooks agree -- but in general USA Today agrees with AP while the New York Times and the Wall Street Journal agree with the Post.
The Chicago Manual of Style is to more formal publishing what the Associated Press Stylebook is to newspapers, and it takes the formal route:
The possessive of most singular nouns is formed by adding an apostrophe and an s, and the possessive of plural nouns (except for a few irregular plurals that do not end in s) by adding an apostrophe only.
(Note that Roberts as a last name does not mean "more than one Roberts.") Chicago gives Burns's poems and Dickens's novels as examples.
Words Into Type has a similar entry.
So does Garner's Modern American Usage:
To form a singular possessive, add -'s to most singular nouns--even those ending in -s, -ss, and -x (hence Jones's, Nichols's, witness's, Vitex's).
The most interesting exception, addressed by some of the above, is for Sox, as in Red Sox and White Sox. The obviously correct solution is Sox', as the spelling is an analogue of Socks (plurals take the apostrophe alone) and, of course, people just don't say "Sox's."
For much, much more on this topic, see "The Elephants of Style."