This is a a really old post, but I agree. This is not okay.
@Byron10 makes a good point that the rent is not the only expense to be considered. It is usually the landlord who is responsible for maintaining the property and insuring it and often several other things. I used to have an apartment that I rented out for a while. There, there were very heavy service charges, which including things like the buildings insurance, hot water and central heating, maintenance of the lift, communal gardens and much much more. With the usual expenses, like the mortgage repayments, which usually include a lot of interest, there was very little profit margin from the rent.
And what about the deposit or downpayment that the owner has put down?
Then there's the wear and tear. How many people are in and out? How are they treating the place? I know that usually there is much more wear and tear from short term guests unfamiliar with a space than there is with responsible long term tenants. Why should the landlord pay for that? What about nuisance to the neighbours?
What if something goes seriously wrong and Airbnb doesn't cover it? Do you think that the landlord's insurance would when the place has not been used for the purpose stated? I doubt it. Then you are really up the creek without a paddle.
I have no sympathy for a tenant who sublets to make a big profit from my investment and liabilities and does it in such an underhand and dishonest way.
That's why it's so important to state clearly in the least that sub-letting is not allowed (standard practice in the UK to include this). I would evict anyone who did this.
Also, seeing as the tenant is not living in the apartment and all and the owner has not given permission for those guests to be there, would they technically be counted as trespassers?