How do I receive my card?
You do not actually receive the physical card and you do not need to provide your address. You are given the code that comes right off the card (unless you request the actual card). This code is redeemed in the Google Play store and it will give you credit on your account.
How can signing up to websites possibly get me a free Google play card code?
Thanks to the expansion of the internet companies are turning towards it for product research, this means they are willing to give you (the consumer) a number of rewards in return for providing your opinion when signing up to their sites.
Is this safe and legal?
Yes this is safe. It is also legal.
Do I need to be in the United States to get my code?
No, you can receive a Google play card code anywhere.