I buy a lot of ice for my vintage base ball team in the summer and I would never dump the store bought ice from one of those garage or grocery store coolers into my beer. I've emptied a couple of them down to the bottom, and you wouldn't believe the garbage I've seen down there, food wrappers, cigarette butts and god know what else. Plus who know where folks grubby little hands have been before the stuck their hands in there.
And most of the bags I've bought aren't completely sealed anyway, there's either a big gap at the top where the sting tie is, or there's drain holes in the bags, or they're just plain ripped,
Store bought ice is fine for cooling your beer if the ice is in an icebath on the OUTSIDE of your kettle, but unless you've made your own ice in a sanitized, sealed container, with preferably boiled water, then I wouldn't contemplate sticking it in my wort.
Just because somone's done it and hasn't had any issues SO FAR, doesn't mean they're not playing russian roulette with your beer. The one way to gaurentee there will be no problem, will be not to risk it.
