A beta doesn't guarantee that these problems would be found. It took over a month with millions and millions of kiddos online to find stuff like the infinite ammo exploit. No QA team can compete with the Jimmy Schoolboys of the world playing 8+ hours a day, 7 days a week.
IW most likely had no input on whether or not a public beta would run. These "betas" on the console side aren't even real betas anyway. They're just public demos used for building awareness of the product (aka MARKETING). Which brings me to Activision (You know, the ones that own the CoD franchise, distribute the games, market them, provide the funding, etc. etc.) Whatever IW does in regards to marketing their CoD games has to come with direct approval from the publisher (aka ACTIVISION). After all, Activision owns the Call of Duty license and they call the shots on how they want to reveal and market their games NOT IW.
But sure, blame Infinity Ward. Viva Treyarch.