There is a fair amount of grey area with regards re-broadcasting live TV.
My bother has a Slingbox set-up on an extra cable box at his house in Michigan. The only purpose of that Slingbox and said extra cable box is to allow me (sat over here in the UK) to watch US cable on my own television. Part of his cable package (and therefore watchable by me) includes channels that show Premiership football live (Fox Soccer).
Using Slingboxes in this manner in not illegal, indeed that is their purpose, to allow re-broadcasting of media content. The difference though is a Slingbox only allows a single distant viewer whereas straming via a website allows multiple viewers.
Sky have tried taking people through the courts before to protect their right to screen Premiership football but in a fairly recent case (a women who used a non-UK satelite TV provider to screen football in her pub) they lost.
http://www.guardian.co.uk/media/2011/feb/03/eu-law-sports-rights