So when Activision decided to raise the price of their games to $60, and other companies followed suit, that only affects the Activision customers? Companies watch other companies for new ways to milk their customers.
It may seem small, but MS may decide to raise the price again down the line, seeing how quick people are to spend their money.
It's not like you are somehow entitled to get XBL Gold for free. Microsoft have been open with the fact that it's a paid service from the start, and since the price has dragged behind inflation, they are now correcting it.
Don't start with that "inflation" BS. It doesn't cost them millions of dollars of subscription fees to run Xbox Live; the only expenses they may have are maintenance of the shop and voice chat servers, they don't even run game servers.