Different lobbying groups represented different law enforcement groups, and those groups received different treatment -- that seems to be the short version.
from the article:
The lawsuit points out that the two groups exempted from the law — state troopers and inspectors — are represented for lobbying purposes by the Wisconsin Troopers Association, which endorsed Walker in the 2010 election. All other law enforcement groups represented by WLEA that did not endorse a candidate in the 2010 election were covered by the law.
The lawsuit said the law forced the WLEA to break into two parts, each with different rights.