Does the page "belong" to the ad campaign, or does it "belong" to the user/company?
If it is the former, then I can understand why facebook would close it when the ad campaign is over. If it is the latter, then it should not be tied to the campaign, and should not be closed when the campaign is over.
In the latter case, if I set up a page about a product/service and decided to pay for some advertising to drive traffic to that page, I wouldnt expect my page to be deleted after the ad spend is discontinued. In fact, I would argue that the ad spend in that case was a waste of money - all the users that I have driven to my page, are no longer able to access that page. As an individual, I would likely seek recompense through the courts in that case. T&Cs don't give you immunity from being a dick - at least in the EU. Not sure what the situation is for companies.