A curious observation. Thanks. I'm not really informed about expensive hotels, almost everything I know is about the middle segment.
> At cheap hotels they use it to attract customers
At the hotel market of the region I know guests wouldn't even consider booking a hotel that doesn't list free WiFi (unless it's really fantastic in other aspects and/or unbelievably cheap) and they would put submit a negative review if they came and there was no free WiFi (even if it wasn't advertised and they've just didn't notice), it's like if there was no free water in the WC.
> but the hotel gets some extra income.
And the guest gets some extra headache with authentication and billing instead of just turning WiFi on and using it.
This is common with a lot of chains these days. Free WiFi is a carrot to get you to book through their site so they save agent fees.
An observation on the side: My travel policy covers a number of "paid movies" depending on the nights travelling. With proper internet that business of hotels finishes and travellers get their pr0n — oh I mean action movies — for free without it appearing on the hotel bill.