tptacek is asking how investors buying properties to rent them out, which clearly leads to increased rental supply, then somehow supposedly leads to higher, not lower, rents.
Recognizing your frustration with reality's failure to adhere to the academic: the fact is that rent rates for corporate-owned units generally don't go down. At least, not in recent history. In the rare cases where cartel behavior doesn't work to cement rates, and owners have to respond somehow to market conditions in order to avoid cash-flow disruption, they will offer "specials" that lower the out-of-pocket cost, but not the on-paper rental rate, for a unit. Your oft-found "1 month free"-type deals (that are actually a monthly bill credit for the initial lease term). Upon renewal, your increase is based on that paper rate, not what you were actually paying.
Increased rental supply at the cost of decreased home ownership.
There is already price manipulation with rental properties. If a cartel is in control of enough of the supply they can set their prices as high as the market can afford. There is already a nationwide shortage of affordable housing in desirable places with jobs and the idea ITT is that it will only get worse as the investment class are the only people that can afford desirable property.