According to https://www.science.org/doi/10.1126/science.abc1507, the variation of other radioactive isotopes in Fukushima wastewater is rather large between tanks: while tanks are on average within legal limits, different tanks may have different quantities of these radioactive isotopes that bioaccumulate in fish.

Moreover research into bioaccumulation of radioactive isotopes is rather limited (because, y'know, people don't usually dump nuclear waste into the environment), so it's really not super well established how fish process these things.

Plus, Tepco's track record of not cutting corners isn't looking very good... But this will save Tepco and the Japanese government billions of dollars, so I guess go them?