The right-wing hacks could be slaves of some defunct economist that they read back in their college days, whose text assumed that everyone pursues their 'self-interest'. But if carefully examined, the examples of self-interest in the textbooks implicitly assume a special form of narrow self-interest: the default case is that Person A gets utility from the goods and services that A alone consumes. No one explicitly cares about what anyone else consumes. In the core model in the textbook, this narrowly self-interested behaviour leads to 'efficiency' and a Pareto optimum where no one can be made better off without making someone else worse off. (A moment's thought can convince anyone that this says absolutely nothing about the desirability of this particular state of affairs, but never mind...)
It seems to be all too easy to slip from examining an unrealistic hypothetical situation that explores what would happen if people were actually like this, to assuming that all rational people actually do behave in this way. Back to Brown's flunkies' claims of hypocrisy against Elizabeth Warren.
'And if Elizabeth Warren is not a hypocrite then she' must be a Commie!!'
If we suppose for a moment that they aren't just hypocritical bullshitters (quite likely, given the pervasiveness of bullshit in our culture) and that they actually believe that Elizabeth Warren is narrowly self-interested, then she can't be advocating policies that would raise her own taxes out of conviction that these would be a good thing. No, she must be pretending to support those policies achieve even greater gains for herself, namely winning a Senate seat and then (who knows?) maybe she could get a $700,000 advance on her memoirs like Senator Brown has received.