When Jeff Rae was arrested last October with hundreds of other Occupy Wall Street protesters during a march on the Brooklyn Bridge, he decided to fight the charges, believing he had been entrapped.

On Monday, Mr. Rae changed his mind and accepted a plea agreement with Manhattan prosecutors. Why? The district attorney's office had subpoenaed his Twitter account, raising the stakes in what he had thought would be a speedy case he could win, he said.

"You're...