Not a big deal - storage volume is perhaps the cheapest of the Internet currencies these days and certainly not a problem for Google. Just imagine the size of storage space Google uses to cache almost all of the internet (count images too!) and it'll become apparent that one two or perhaps even twenty or whatever more Gb allocated for each Gmail user is just a fraction of storage capacity Google uses for its main business. To decommision some outdated hard disk arrays and use them for an auxillary service is only logical and comes at almost no additional cost.
It's not really fake - it's allocated to you. As with bandwidth or CPU resources it supposes that not everyone and actually nobody would be dumb enough to use it to its full capacity so the actuall storage space used for Gmail is times less than the allocated amount in Gb multiplied by the number of registered users. Now after you've mentioned this I feel terribly tempted to stuff my Gmail account with some crap up to the neck just to see what will happen 