Sorry mate I had a re read, and edited my post, originally thought all HD was missing with glitchy SD
then noticed it was just Sky Arts, so see post #2 again
Example: Server 1 buys legit subscription for limited services, say
standard package plus sports.
He shares the sports pack with server 2 who has BT sports pack.
He shares the sports pack with server 3 who has Films pack.
He shares the sports pack with server 4 who has Porn pack.
So his sports pack gets diluted and his overall package gets bigger at a reduced cost.
But decent ones tend to keep the main ones as "locals" (not shared) and share lesser channels.
Same as a PPV events, they don't all pay for individual subscriptions on the night, some share.
The above example is actually a bit more technical, but you get the idea.