This is what the Steelers do. They don't negotiate with players not in camp, except for rookies. When Hines Ward held about 6 years ago, they refused to negotiate a contract extension with him until he reported.
Mike Wallace has few choices:
Show up to camp and work out this long-term contract.
Sign the tender, work on a long-term contract, and worst-case become an unrestricted free agent after this season.
Sit out, get stuck as a restricted free agent again, and go through this entire ordeal again.
But the only team that he can possibly play for this season is the Pittsburgh Steelers unless they choose to trade his rights to another team.