I don't think you can blame that on Britax themselves. I've never seen them advertise themselves as "the safest," and their FB feed, for example, is mostly full of tips on proper use, which are accurate. Sure they promote the safety features of their seats, but so does everyone.
Britax seats are relatively easy to use properly, generally well made, and their customer service is good and not sketchy. Those are the kinds of things that inspire brand loyalty in people. Some of those people are in "the bubble," and yes that's annoying, but lots of others just like them because they are nice seats.
I have seen other manufacturers claim their seats were "the safest."