The real reason behind this fine is the heavy lobbying from the big Hotel companies in Spain, they are specially making this pressure at a local government level, as they are going to be the regulators (The new renting law differentiates between short and long term, and gives the regulation powers to the Comunidades Autonomas).
Neighbors horror stories like the ones we´ve been seeing in NY, don´t seem to be as big a factor in this. Still it happens, mainly in the big cities like Barcelona and Madrid, were some flat owners are having problems with the guests of their neighbors. But I´m not aware that the neighbors are too vocal (I´ll gladly be corrected about this).
The Hotel lobby is seeing how a big chunk of their business is going to the share economy and they are afraid.
In Mallorca where I live, most AirBnB offerings are mostly for summer apartments and homes (not at the city), so there is little problem with neighbors (who mostly want to do the same). People of course see AirBnB as a way to keep low the costs of owning a summer apartment (renting the weeks they are not going to use it). This is specially interesting to those who are trapped on high Mortgages from the bubble, as it´s providing them another income source. People who inherited or bought before 2004 are seeing mostly profits, and if you are lucky (and dedicated) you are able to make money of it.
So far the legal status in Mallorca is good, as you can register your apartment for season renting only paying a yearly fare per bed to be rented. But they are certainly receiving heat by the lobbys and this could change any moment.
I guess that the best move by AirBnB is to start selling this local governments the advantages of AirBnB:
-to improve the usage of the empty apartment stock (that eventually increases the plummeted property prices)
-Putting money on the hands of the people who more needs it.
-Not a killing competitor to hotels, while certain % of clients will move to AirBnB, not all people wants to go to apartments (even less if they are privately owned), for they vacations. It´s a similar concept to the low cost companies, they increase the total traffic, and the total business for the area, it´s not only the same passengers traveling cheaper.
Don´t need to be all local government, just 2 or 3 of the most touristic ones like Baleares, Canarias, Valencia, Cataluña. Once one starts making AirBnB friendly regulations (and selling them as an advantage for the government itself), most of the rest will follow.