In my last job slowly got rid of all the youngsters in favour of middle aged part time mothers. Productivity went through the roof! Very few sick days. They worked compressed and flexible hours, often from home. Never lost a member of staff to a competitor. All our clients were delighted.
Yes they do have a contract. This is a large company. I'm specifically talking about the youngsters who come in and talk to each other about their "wild" nights and how they couldn't get into work last week because of it and how "John doesn't care" (obviously not the store manager's name)
The issue here is not that the staff who don't get sick pay get away with it but that your store manager isn't being fair regarding disciplinary procedures. Your company should have a sickness policy that is followed by everyone regardless of their sick pay.