The pros who are in the gruppetto/laughing group always have to ride hard enough to make the time limit on every stage in a multi-day race, so even if they were tempted to get off the bike, they wouldn't. Plus---they're far more likely to make it to the end of the stage inside the time limit by riding in the gruppetto than if they stopped and then rode the final kilometers alone.
By the way, Chris Horner didn't say that he stopped at a picnic area---he said that, after riding at his limit for Cadel Evans on that infamous Tour stage where Floyd Landis did his (literally) incredible ride (recouping almost all of the time that he'd lost in the previous stage), Chris pulled off the road and had a private picnic with his Snickers and Coke from the team car before getting back on the road and finishing the stage (still well ahead of the gruppetto, no doubt).
[Edit: I see that Horner mentioned in today's Butterfly Effect video that it was indeed a picnic area where he stopped to enjoy his Snickers and Coke. Sorry! I believe he didn't mention that detail in his full-length video about that stage, which he uploaded to YouTube sometime last year. Great, great video, if you haven's seen it.]