"For every complex problem there is an answer that is clear, simple, and wrong."

Is $20,000 a hard number. What if you make $20000 this year and would like a raise. That would automatically be detrimental, forcing you to either not take it until that raise is >20% over $20k. Then since is rent money, and you would have to income taxes on this additional money over $20k you would actually need 20% + 15% over 20k just to break even on what you were saving in rent.

Usually with caps like this the break is phased out. So if you make between e.g. $20k and $25k, maybe only x% of your rent is paid for, where x is between 0 and 20.

So, an individual buys a one family home, at a convenient location, with the intent of renting it out as an income-producing property. The person pays extra money, since a more desirable piece of property is more likely to stay occupied and produce rent.

The owner gets it ready for rental, and selects a rental price. The rental price covers the costs of buying the property, improving the home, paying taxes, and a percentage to ensure repairs and upkeep over time, with the hopes that a portion of the money dedicated to upkeep will eventually become profits, depending on tenant churn.

Someone shows up seeking to rent, and just happens to make less than the magic number (under 20K for example), so the owner has to forfeit the portion of the price that covers taxes and upkeep.

The owner is expected to make a decision that will turn a potential source of income into a potential burden, which risks falling into disrepair. How then is this decision expected to play out?

Income level isn't a protected class in the Fair Housing Act, so it's possible to just refuse to rent to such people.