i have just read that stocks and shares isas benefit only people who pay 40% tax.
Can anyone explain to me why this is so.

Actually if you take the CGT example then a stocks ISA wll have more of a benefit to a 40% tax payer given the CGT is charged at 28% not 18%. So a 20% tax payer investing the 100K outside an ISA would pay less than a 40% payer in total CGT. It is arguable whether benefiit is the right word since inside an ISA neither pay any tax so in that sense they are both equal as far as treatment is concerned. In pure money terms the 40% payer has saved more actual cash from the taxman but then it was his to start with.

Depending on which party you support you can either decribe this extra saving in terms that make it appear the government is giving away money, having taken it away from the poor / deserving or you are simply letting people keep what they've earned from their own hard work. No doubt there will be disagreement as to what hard work is and at what level of income it stops being hard.