When you read the context carefully, it becomes evident that Romans 6:7 is really saying that the dead are free of sin's power to control their bodies. It's not talking about sinners being acquitted for their past sins. It's actually saying that the dead cannot sin - because there is no living body for sin to control - and using this as an illustration to tell christians to likewise consider themselves as being dead and therefore not to allow sin to control them.
Watchtower is shamelessly twisting the verse completely out of context to support its false teaching about the unrighteous getting a second chance. Watchtower's definition of the unrighteous is completely made up and has absolutely no basis in scripture. In fact, scripture implicitly contradicts the notion that the unrighteous is a separate group from the wicked:
"What! Do YOU not know that unrighteous persons will not inherit God’s kingdom?" - 1 Corinthians 6:9a
"Jehovah knows how to deliver people of godly devotion out of trial, but to reserve unrighteous people for the day of judgment to be cut off," - 2 Peter 2:9