The underlying problem is that it violates the owner's property rights. Whether they rent or or leave unoccupied their property, even if it's scarce, is their business. And not letting the owner manage their property as they see fit causes all sorts of wider problems.
I'm assuming from reading your other comments you're not going to be sympathetic to this argument so let me give you two anecdotal incidents.
The first is about a woman who is 70 years old. She lives off a state pension which is supplemented by a small rental income from a three bedroom apartment she owns. The apartment is quite old, and needs refurbishment, so she can only generally rent it to students or non-professionals who pay a modest rent. After maintenance costs, property tax, and building management fees she doesn't get much but enough so she doesn't have to rely on her children.
One day, one of the tenants stopped paying rent. Rather than asking him to leave immediately she gave him some extra time to get the rent. Rather than do this, he decided he would do an occupation. So first he terrorized the other tenants so they left and then proceeded to occupy the entire apartment, including turning one bedroom into a gym. Police were called quite a few times but they said they couldn't do anything until they received a court order. So for a year and a half, this guy lived there rent free with water and electricity being paid for as well (you can cut off internet as it's not deemed essential).
When the court order finally came through the police didn't even bother showing up and it was the locksmith who chased the guy out. He hasn't faced any consequences and could continue to do this again and again. As for the owner, she had to take out loans (some with 20% interest) cover the costs this guy incurred. She, who is Spanish, will never rent out to Spanish people again.
The second is about a taxi driver in his late 40s. After a long period of saving he managed to save enough for a deposit to get a mortgage on a costal apartment that his family would use in the summers and rent the rest of the time. While he could have just listed it on Airbnb, he decided to rent it out to a woman with children on a long-term basis (non-summer months).
So she moves in and when he comes a few days later to collect the first month's rent he's informed that she won't be paying rent and because of the children it's going to be impossible to kick her out. He tries to negotiate with her over a few months, including talking to the town hall to get some some rent stipend, all to no avail. Eventually, he had to use desokupa services to get her out but not before this woman had caused significant property damage.
He also went into debt and his marriage almost failed because of the stress and financial strain. Once he finishes repairing all the damage (which he has to do himself as he can't afford to pay someone else) he will rent it out again but only through Airbnb to foreigners.
So this policy has ensured that honest people, renters and landlords, get punished by dishonest people who won't see any consequences either.