Yeah, that's a good point. Maybe it states on the can of Brak-Kleen if it is harmful to rubber O-rings. I use it on my bolt action rifles to clean the bolt assembly and the trigger assembly and because it is an aerosol, the power helps blow out any of the oil and or dirt/dust from the trigger assembly which you won't get by using Zippo by just pouring it down into the trigger assembly.
Just went to CRC's website and found this info regarding rubber seals. In short, they say it is safe for use on rubber seals, however on the basic specs info, it does say that it is not safe for plastic. If it were me, regarding what cheaptrick stated on use for cleaning AR's, I would just simply follow-up using any type of high grade oil, such as CLP, EEzox, Rem-Oil etc. CRC Brak-Kleen