That's why
subsidies bad and
subsidies good are a false dilemma. "Spending money well" might seem like an obvious solution but it runs up against the problem of corrupt politicians, of which there are many. Hence, we have (worldwide) politicians handing out money to places they are (probably) needed (in your example, the agriculture subsidies) and places they are not (Walmart and similar McJobs), and it's possible that the latter receive more than the former.
It's also possible that it's impossible to get subsidies spent in a way that isn't rife with such corruption. I would say that the only way it may be possible is small government. Not because subsidies are bad, but the only way to watch government closely enough to keep corruption low is to make it small enough to watch in entirety.
Like a computer program, if it has less lines and less features, or less overall complexity, it should be easier to spot bugs and egregious mistakes. That's not the same as not wanting features at all. Or, it can become a political version of Zawinski's law where people want more from government and the resulting size and complexity is such that a group of corrupt developers can spread their corruption across so many branches and parts of the project that no one can watch it all (after all, isn't that how the 3 letter agencies undermine security software?)