Best option is pretend you never saw it, and don't look so closely in the future.
Yes you can kick up a stink about it, but what good will that do you?
Quite often the businesses that do this are small and if they were to have to purchase the licences they'd have to take that out of someones pay. So if you like getting paid, and the job is otherwise OK, don't make an issue out of something that is not your problem, nothing positive can come of it for you personally.