In his book John Kenneth Galbraith: His Life, His Politics, His Economics, Richard Parker tells us that Galbraith's first specialty in economics was agricultural economics. As a result of earning his Ph.D. in ag econ at Berkeley and having the right connections, Galbraith ended up teaching ag econ at Harvard in the early 1930s. Here's how Parker describes Galbraith's approach to his research in the area:
Galbraith's audience might be economists, but his constituency was the American farmer. Implied throughout (when not explicitly stated) is the belief that stable incomes and reasonable profits for farmers and an ongoing central obligation of government to assure decent incomes and profits (not just when markets fail) should be agreed on.
In other words, Galbraith was working to promote the interests of farmers, even when markets weren't failing. His goal was to make sure that farmers had "stable incomes and reasonable profits." It's not surprising, therefore, that he advocated price supports for agricultural crops.
Because Parker is such a fan of Galbraith and possibly because he seems to have, at times, a tenuous grasp of economics, he doesn't seem to think that he needs to justify Galbraith's approach. He doesn't even seem to think that there's an issue here that most economists would immediately wonder about: how do you justify transferring wealth from the rest of society to farmers?
Government policy to assure farmers' incomes imposes deadweight losses on taxpayers, consumers, or both (depending on the form the policy takes). Why should farmers be a favored constituency? Parker doesn't say. More important, he doesn't give any idea of what was going on in Galbraith's mind that led him to tilt his research toward a special interest group. Maybe it was simply the fact that Galbraith grew up on a farm. But one thing that economists typically learn, no matter what program they're in, is to put aside narrow parochial interests and think about the general welfare. Galbraith didn't seem to do so. Why? It's a puzzle.