The monetary amounts move the decision psychologically to a new category. This new category typically triggers economic weighing of costs and benefits. Such weighing takes more mental effort than the kind of hair-trigger evaluation a marginally-costless action (like a comment upvote or Flattr-click) does. That mental effort is transactional friction, a tax on seriously considering the transaction. At the margin, some people move on to the next shiny link instead of engaging in the weighing effort.

The motivation-crowd-out effect is related, as is the "don't insult me by putting a measly price on this" sentiment (which I think dominates your doctor example) -- but the theory about previous micropayments that I believe true is in fact talking about a decisionmaking friction.