most lfs have thier tanks plumbed together... and if they have ich in one tank, it is in all thier tanks...and chances are, like discussed before, ich is everywhere... there is really no way to get rid of it...
the best chance of 'getting it' before it becomes problematic is direct from a wholesaler as most o fthose places use coppper in thier acclimation systems..., but they really dont do a real long/thorogh ich treatment...
also, some fish are way more prone to certain deseases/issues, and tang, especially blue / hippo are VERY prone... and unless you have an ich free system, (read above), then chances are your tang will get ich also... depending on tank conditions/tank mates/diet, etc....
the only tihng i would be pissed at the lfs for is for not discussing this inforally with yoyu to let you understand and know ...