I think it does depend a little on the context. People need to be able to rent housing; even if we all had the money to buy, we wouldn't necessarily choose to, for a variety of reasons.
I have been a private tenant for a number of years now as we try and save to buy a house. Is it frustrating that those who own a home can afford to buy the cheaper houses then then rent them out? Yes, very! Is it immoral? Not necessarily. A good landlord is an assest.
I feel that boundries are crossed when landlords exploit their tenants; either by profiteering, or not being mindful of their obligations. By buying a house to rent, the landlord should understand that they will be obligated to their tenants, and not simply raking in the rent each month. They need to provide a certain standard of facilities. Too often I have viewed homes with ill maintained facilities at rediculous rents. That is exploitative.
Additionally, private tenants have a right not to be harassed by letting agents charging rediculous fees for everything under the sun - even for extending your tenancy?! Also, the carrying out of constant inspections. I saw recently a letting agent advertising to prospective landlords with the banner: "Now with quarterly inspections". Really? Tenants do have a right to a private life! Not everyone is careless and likely to cause damage. You are assumed guilty before you step foot in the door.
Don't get me started on developers that buy properties and break them up in to bedsits. Sooner or later some communities won't have any small, affordable, family homes because of this sort of thing.
Sorry, that was a bit of a rant, but private tenants are exploited and are viewed by some landlords and letting agents as if they are less worthy of a decent standard of housing, and privacy, than a homeowner.