My family and I know a lot of people, so inevitably there is usually one birthday party to go to each week.  Whoever is having the birthday buys all the food and beverages, then when it's our turn we do the same thing so everyone comes out equal.  However, we do have a few friends that own bars and every time they have a party, it's at their bar and everyone ends up buying drinks at the bar, no discounts, no beers on the house, nothing.

Is this just a Phuket phenomenon, or is this a widely accepted practice to milk your friends and bring in some extra cash for some free fried rice?

Perhaps you need to examine your definition of the word 'friends' ?

You buy on your birthday? wow! I'll come to your BD party but I am celebrating mine alone!

Is this just a Phuket phenomenon, or is this a widely accepted practice to milk your friends and bring in some extra cash for some free fried rice?

I don't think it's limited to Phuket, and not just when it's the bar owners birthday, but also when it's their wife/girlfriends b-day, the manager's/mamasan's b-day and who ever else's b-day that might attract more than just the few hard-core regulars.