The issue isn't about impeding the goalie. The issue is every team in the league have had their goalies impeded by Arsenal players this season, and nothing was done about it. They allowed those grapples, pushes, shoves and thereby set a precedent on how corners and set pieces were to be officiated this season. Chalking a goal off for impeding the GK, after setting a precedent to the contrary, on the most important game of the run-in and thereby deciding the title and relegation is why people are angry.