I have heard others say that, even in my local area, and I’m not really sure where that notion comes from. Physcians are no more liable signing a Basic Med form than they are any number of medical waivers (such as for motorsports or even CDL medicals), and I have had absolutely no problem getting my PCP to sign the BM form every few years. The BM form even specifically says that it’s the pilot who ultimately is held responsible.