Warning - MPS Junk mail opt out only lasts 5 years!

Having just registered with the UK's MPS (Marketing Preference Service) to stop getting junk mail I got a confirmation letter warning that it will expire in 5 years, and that it will take them up to 4 months to stop their marketing association members from junk mailing me. Not very effective+efficient is it.!? Why can't it last for ever?