To you, you are working for free...
To the company you are working for, even though they aren't paying you any salary, they probably still need to spend time training you, giving you access to their information etc... which means there's still a cost to them...
To the company you are working for, even though they aren't paying you any salary, they probably still need to spend time training you, giving you access to their information etc... which means there's still a cost to them...