People who use low cost tricks to protect their property only put themselves at risk.
In the same way, the fake security systems will cause increases in the relevancy of the failure rate of real security systems. Let's say that we have 100 criminals, 100 homes with 98% effective security systems, and 100 homes without. Currently, the 100 criminals will rob the 100 unsecured homes, because they don't want to take the 98% failure risk. Now, let's say that the people in the unsecured homes put in fake systems. The 100 criminals now attack 50 secured homes and 50 pretend secured homes. The security system stops 98% of the attacks on the secured homes, leaving one of those attacks as successful. The fifty criminals who attacked the unsecured homes still get away with everything. Thus, one person who bought a security system is attacked who wouldn't have been attacked if the fake systems weren't in play, so the people with the fake systems have added to the risk of people who own real systems. However, we have also decreased the number of successful attacks, so I'm not sure that there's reasonable claim of moral wrong-doing in this scenario.
Now, my numbers are obviously fictional, but the fake systems will increase the number of successful attacks on real systems. Whether that's a bad thing is an entirely different issue.
Eventually, security system warnings will lose their deterrent effect, and people with security systems will be attacked more frequently than they are now.