Boffo box office for the R-rated “The Matrix Reloaded” may be cause for celebration, but the film’s huge success says something less encouraging about the difficulties of enforcing the Motion Picture Assn. of America’s ratings system.Hollywood could not ask for a more impassioned and thoughtful advocate than Jack Valenti, but his defense of the 35-year-old ratings system is increasingly unconvincing because it can’t be adequately implemented. Take “Reloaded.” Warner Bros. reported that 50% of opening-weekend crowds were under 25 and that for many folks the film became a family event. Producer Joel Silver reasoned that “the audience rated the movie.” Exhibitors, meanwhile, just two years removed from a bitter feud with Washington over kids’ access to violent fare, simply threw up their hands. Given how many screens exhibitors pack under one roof, enforcing MPAA ratings is nearly impossible. If the studio positions “Reloaded” as a family film and theater owners lack resources to make sure those under 17 don’t get in, how credible can the ratings system be? This phenom is set to repeat itself all summer, with the release of R-rated titles like “Bad Boys 2,” “Terminator 3,” “American Wedding” and “Gigli.” While we believe filmmakers have the right to make edgy and challenging works — and inevitably they will try to get them in front of as many ticket-buyers as possible — we think exhibitors, in concert with the MPAA, need to be held accountable for whom they allow into restricted movies. If they aren’t, D.C. politicos could decide to take on this summer’s slew of R-rated tentpoles for their own purposes.