Why bother justifying it? The simple fact of the matter is that rent is consensual, and taxes are not. Hence, rent is justified, and taxes are not. Claiming that they are the same thing falls apart after the smallest amount of consideration. Rent is part of a contractual agreement, taxes are not (the "social contract" doesn't exist, and even if it did, the state is the sole party with the ability to change the terms of the contract, and does so without the agreement of the taxpayer), so the standard punishment for not paying wildly differs between the two.
The most common reasons I've seen for complaining about rent are notions of fairness or the amount of work a landlord has to do to make money, with the image you posted falling into the latter ("hard earned income"). Work has no inherent value, so this doesn't actually matter, but in a practical sense, people that think landlords can just sit back and earn revenue are sorely mistaken. There are many obligations to fulfill, and if your property is in a shitty area, for instance, the likelihood of your tenants not following through on their contractual obligations is relatively high. There are plenty of other contractual obligations that have to be fulfilled, too, with this obviously varying depending on location and such. The idea of landlords sitting back and just getting rich is a leftist fantasy.