Fair point, but there's a common trend with all three teams: poor three-point shooting. Cleveland's got a postseason-low opponent 3PT% of 28.1. Couple that with the fact that Cleveland's opponents are even shooting worse on wide-open 3s and it seems that these underwhelming offensive performances are largely due to Cleveland preventing them from getting the kinds of three-point looks they want.
There's another explanation here, too, though:
Cleveland hasn't faced a single opponent that is particularly good from the perimeter.
The Celtics jacked threes all season long but were near the bottom of the league in perimeter shooting effectiveness, so that's easily explained.
The Bulls have never been known as a great perimeter team.
The Hawks were a 60 win team in the regular season because of their excellent three point shooting. However, injuries to Demarre Carroll and Kyle Korver undercut their perimeter prowess pretty substantially. Without Korver and Carroll providing the shooting balance on the wing, Teague, Millsap, and Horford cease to be particularly effective floor spacers.