Apple's iPad OS licensing agreement shows that users will only get one free major OS upgrade and will have to pay for any further upgrades.
Apple will provide you any iPad OS software updates that it may release from time to time, up to and including the next major iPad OS software release following the version of iPad OS software that originally shipped from Apple on your iPad, for free. For example, if your iPad originally shipped with iPad 3.x software, Apple would provide you with any iPad OS software updates it might release up to and including the iPad 4.x software release. Such updates and releases may not necessarily include all of the new software features that Apple releases for newer iPad models.
iPhone users get free OS upgrades, while iPod touch owners have to pay for all major OS updates. Apple claims that this is down to accounting practices. It's unclear what the reason is that iPad users will get one free major update and then have to start paying for updates.
Apple is a master at nickel and diming customers, so this sort of move is not an unexpected one.