Exactly the same thing that's stopping the landlords from raising the rent by $300 right now - supply and demand. If the people receiving UBI are universally s

Exactly the same thing that's stopping the landlords from raising the rent by $300 right now - supply and demand. If the people receiving UBI are universally spending that money on something other than rent, then the landlords who raise their rent by the UBI amount will become noncompetitive and fail to attract tenants. To presume that 100% of the UBI will simply be absorbed by the landlords is absurd, not only because of the above argument, but because for many people, the UBI will be significant enough to swing them to home ownership instead of renting, putting additional pressure on the landlords not to raise prices.