I do not think that it is prejudice against WalMart bikes but prejudice against junk that tries to masqureade as bicycles. Low end WalMart bikes may be the worst but Target and other mass merchandisers sell junk masquerading as bicycles too. IIRC Huffy and Murray bikes received similar treatment at bike shops long before WalMart, and their "bikes", became ubiquitous.

Many bike shops have always preferred to not get involved with mass merchandiser bikes due to their lack of quality and the inability of their bottom end components to stay in adjustment for any reasonable time. Working on them is like trying to polish a tu-d and has only gotten worse with dual suspension so called "Mountain Bikes" from these merchants.