Ick is usually present in most systems (it is very hard to achieve an ick-free system) unless you are running cooper or hypo. Ick will not usually progress to troublesome proportions if your fish are happy, well-fed, and otherwise stress-free. New tank additions are normally pretty stressed (put yourself in their shoes!). Even though some holding facilities and many LFS (I know Tanks A Lot does) run cooper in their fish systems, you might still get ick in your tank from that new addition. It's not so much that the fish had ick but that you probably already had the parasite in your own tank but it was not troublesome due to low populations and the fact that your existing fish are able to fight off the intruders just from normal healthy immunity. Now the new fish doesn't have that luxury. He comes in your tank, stressed out, and then maybe bullied by your existing fish and suddenly his immunity is not sufficient to combat the ick that was already present in your tank. This new now infected fish will explode the parasite’s populations and this might be too much for some of your existing fish to handle. If your water parameters are a bit out of wack then things might just spiral downhill quickly.
Want to never worry about ick? It's not easy but here is how you do it. Remove all fish and treat in a separate hospital tank for a minimum of 1 month (2 would be ideal since some variants of the parasite have longer life cycles). Note that the only 2 bullet-proof methods of eradicating ick are hypo and cooper. Run your tank fallow for the duration of the treatment of the fish. This should pretty much get you clean but now you will have to take the precaution of quarantining and treating any new fish additions with hypo or cooper for at least 1 month.
In the interest of full disclosure, I have not followed this advise myself and because of it, I've had my share of losses over the years due to ick.