I dunno how it is in the US, but over here there are two types of sick pay, statuatory and company.
Statuatory sick pay is paid by the government and is quite minimal.
It also has to be accrued via tax payments on you weekly/monthly pay, and even then, you can only take so much in a year!

Company sick pay is completely voluntary by the company at hand, with the company under no legal obligation to provide anything!