I don't disagree that landlords are for the most part acting parasitically. However I would argue that in order for society to function "parasitism" is a requirement. I want to be clear and state that THIS form isn't required, but some form is.
Let me explain my thinking. Nearly half of the population doesn't work. The population of non workers can almost entirely fall within these categories: children, attending school, disabled (mentally or physically), or retired.
These populations need money even though they are not producing any. I would guess that most of the extracted profit that comes out of "mom and pop" rentals goes to providing for non-worker expenses.
Now I believe these expenses should be covered by taxations and redistribution of the factor income, but since we have a pathetic system of this in the US it's hard for me to fault someone for using investment property to hedge against child care and/or retirement
But my point is that there are certain categories of people who need access to income that they themselves do not produce. An able bodied capitalist is not one of these people, but a worker who is using real estate as a retirement vehicle is.