Planetary magnetic field only weakly protects against cosmic rays (extra-solar origin).

A thick enough atmosphere will stop pretty much all the charged particles from the normal solar radiation.

If it would be so bad, Earth's polar regions (experiencing aurora borealis) would be inhabitable too. Earth's magnetic field is not magically neutralizing all charged particles from the Sun, just diverts them (some maybe away, but many simply towards poles).

And clearly even our mag field (and Sun's heliosphere) is not enough to shield us from those crazy cosmic rays.