Norton Internet Security 4 subscription does not expire

Hi, 

 

i bought NIS4 two years ago.I've been using it until October 2009 (because the problem with version for Snow Leopard).

Few dais ago i installed the NIS4 again and it works , upgrades and updates with newest definitions.I know probably it is

a stupid question because it is good for the customers but why NIS4 does not require renovation and serial?As i understand it counts on the fact that if i don't reinstall my Mac OS in one year it will expire.If i do it more frequently no problem.Please explain.