I would guess they knew what awaited them, but hoped efforts to open the country would be successful and they would be allowed to return. The Manhattan was able to return some sailors to Japan in 1845[1] that shipwrecked in Japanese waters, making the rule seem somewhat flexible.

I think an earlier version of the Perry expedition helmed by John Aulick intended to try and return 17 Japanese sailors as a sign of good faith/bargaining tool.[2]

[1]https://en.wikipedia.org/wiki/Manhattan_(1843_ship)

[2]https://www.classicsofstrategy.com/2015/07/commodore-perry-o...