[jondubois]

That's true and also they don't always rent out the houses/apartments that they buy; if someone wants to launder a lot of ill-gotten money and buy many apartments/houses, it may be too much hassle (and draw too much attention) to deal with tenants and real estate agents so they just leave them empty, wait for prices to go up and then sell for a low-tax capital gain.

They don't necessarily need to receive rent payments to 'clean' the money. It's possible that what is regarded as money laundering in their country of origin is not regarded as such in the country where they are buying the property.