Yes it is what Sky has imposed in their model they may not admit it but there you go they are the content provider for their sports.
There's a thread here about the 3pm blackout
When you look at the other link I posted you can see why the EPL are not interested in lifting the blackout in the UK.
Things may change when the auction for 2019 on is done and dusted as there are a lot of new big players showing interest in bidding for the rights to the EPL, Soon know who won early next year lol