Re: What if someone uses it to download copyrighted movies or child porn?
suppose that handles my concern too, but the people who send out black helicopters may not understand the distinction between username and IP address. This 'feature' doesn't seem to work on my own-bought wireless router/modem.
O...K. In for some explaining: these routers broadcast 2 different networks, with different APs, and different IP spaces. One is yours to fiddle with, you can encrypt to your heart's content and it takes precedence in the case of a bandwidth limitation. The other is managed directly by your ISP, is open to all connections but requires a webpage-based login (using credentials valid with the ISP). It also only uses "leftover" bandwidth, for which you are, quite obviously, not charged.
Whether you like the idea or not, it doesn't draw any significant power (I would estimate in the milliwatt range) and should not impact your traffic speed.
It is also operated directly by the network operator (here, the ISP) and thus completely unrelated to your account AND your IP, no black helicopters for you.
In some cases (e.g. Fon), non-subscribers can connect on a pay-per-minute basis, and the hotspot "owner" can choose to receive some of that money (as for me I didn't bother giving my Paypal ID to receive what amounts to pennies; still would pay more than the added 'leccy bill though).