The point is that they may have a lot of unwinding to do before they are relaxed enough for serious insight meditation. Otherwise they may be adding doing on top of doing, which is like trying to play the piano whilst still engaged in armwrestling yourself.
Sometimes we can get caught in thinking there is only 'doing' or 'not doing' in meditation. We may think in the Just Sitting meditation that we are 'not doing' but this is not the case. Perhaps the most important element of meditation is the 'undoing' (undoing our hindrance activity or 'selfing') - it's not really just a 'doing' or a 'not doing'. This element of 'undoing' means that relaxation is definitely not a neutral state.