A few exemplary reasons
1. It is landlord's business model.
If there is a demand for rooms available to SINless people, there is a supply.
Landlord might run a "legit" business renting rooms to SINless for cash or maybe
untrackable goods to evade taxes (if this would work in this setting of course).
Or maybe he runs a kind of criminal organization, providing a place to stay in
exchange for them working as his goons. He could also force people he
accommodates to work in a sweatshop of some kind, think chinese imigrants being
smuggled to USA and forced to work by triads. Desperate people are easy exploit, think what criminal organizations do with desperate people, some of it could apply here.
2. SINless person is valuable
Maybe they have bounty on their head, in that case landlord just sets up a trap. They can also have valuable skills, maybe landlord lives and rents room in a
"less friendly" part of the town and could use a bodyguard or qualified
electrician and providing them with room is cheaper then hiring. Maybe landlord
wants to use the protagonist as a leverage in some situation or harvest their organs. He could be also be paid by some mysterious benefactor to rent a room to the protagonist (again, if it makes sense in your story). Maybe protagonist have something landlord wants, and they plan to steal it. Of course some of those require landlord and protagonist to be tied in some way.
Maybe protagonist is landlord's friend or family, maybe landlord owes protagonist a favor (or someone else who demands them to rent protagonist a room) or maybe they are forced to, by protagonist or someone else.
4. They are a good person
Maybe landlord is just a good person, maybe their religion demands from them to help those in need or it's just their moral code. They could even run a free shelter for SINless people. It is as good reason as any other and it could be used in interesting ways.
5. They are a bad person
Landlord is a serial killer that targets exclusively or mainly SINless people, as it is easier to get away with murder that way. And how could he say no to prey literally coming to him. They could also conduct illegal experiments on captured SINless, reason is the same, it's easier.
Hope i helped.