As someone who rents out rooms in Lisbon (not on AirBnB), I can assure you the minimum is not 300/month, nor are those rare. A five minute search finds multiple available rooms for <250€ - often with expenses included.

I do agree there is cause for concern, but the knee-jerk attacks on AirBnB is silly. For example, there's not a single word in that article regarding the concerns of landlords in making long term rentals. When it takes two years, hundreds of euros and many lost working days to evict a non-paying tenant, is it any wonder that people prefer short term rentals? When you have a building falling to pieces after decades of rent-controlled tenants paying ridiculously low amounts (I know people paying 30€/m for a three bedroom apartment) and you can't afford to fix it, is it any wonder that you'll sell to foreign investors who can?