I'm travelling abroad and tried purchasing a game, running into a "you're in the wrong country error message". Here's what Steam Support told me:
Our records indicate that your purchase was declined due to your current IP address differing from your billing information.
To purchase content from our storefront (games, DLC or software), you will need to use the following link through a web browser (Internet Explorer, Firefox, Chrome, etc.) to complete your transaction.
For in-game item purchases, You will need to use the following link to add funds to your Wallet.
After adding funds to your Steam Wallet, you can then complete your content purchase through the in game store.
Please bookmark these links for continued purchasing while you are out of the country where you currently reside.
Why is this restriction (IP-based payment blocking) in place? It doesn't seem like it could stop any illegitimate actions (because there's a URL workaround).