They do it because the Republicans lie down and let them get away with it with no challenge whatsoever.
So you are basically accusing Republican poll watchers of looking the other way? Do you even know who they are? For the most part they are just ordinary citizens.
GOP legally barred from fighting vote fraud
http://www.wnd.com/2012/11/gop-legally-barred-from-fighting-vote-fraud/
The case is the Democratic National Committee vs. the Republican National Committee, originally from 1982.