I'm not entirely sure what you're saying.

Diminishing marginal utility of money applies on both sides of the equation to the same degree. To whatever extent this motivation strategy is prejudiced against 'poor people' the disadvantage is mediated by psychological profiles associated with that class, not by the micro-economic incentives present. This does not mean that the problem is unimportant but it is important not to conflate the two. This comment is a (minor and entirely non-offensive) misuse of moral authority.

NOTE: I'm not advocating financial commitment contracts for poor people. I'm not advocating financial commitment contracts at all, for anyone. They do work for some people but I know my psychology well enough to know that they have a toxic influence on me personally if I try that style of influence on myself. People can and should do whatever works for them. But I'll leave championing commitment contracts to someone who likes them. (I'd rather champion, say, self rewards strategies for ugh field removal.)