When will companies stop falsly advertising HDD space ?

You buy a HDD or computer / console it states its an 80GB HDD, size is an example Yet when you turn the thing on or plug it in. You never gives the proper stated amount, mostly around 20 GB is taken off for the OS or for the plug and play aspect of the HDD the plug and play aspect is less than 20GB but you still dont get the full amount paid for. Why don’t the manufactures add an extra 10 – 20GB so we get the proper 80 or whatever size you buy as free space, or class it as is with the OS GB taken off .