It's easy to write a universe program U that would make A cry. The post only proves that A won't end up crying for one specific U, and outlines an argument why it won't cry in some other similar problems.

The class of universes where A doesn't end up crying is supposed to roughly correspond to the informal class of "fair" decision problems where the agent's action is the only thing that determines the resulting utility, but I have no strong argument why it's a good formalization of that class.

One example of an "unfair" decision problem would be a universe that rewarded you for having a specific algorithm, rather than for returning a specific value. Such "unfair" problems can be made up to punish any decision theory you can come up with, so they're probably not a good test case.