Most router's software will log the computer name, MAC address, and much more of the systems that connect to it.
There have been people before that got convicted of this, and it can happen again.
As FabianN says "Be warned! Ye who useth my WifI"
Is it still illegal if say.. you live in an apartment above a cybercafe and leech of their free wifi?
(This is hypothetical! I do not live above a donut shop)