Apple changes their terms and conditions from time to time to which you must agree every time anew in order to be able to access your purchases from the appstore. The current version is to be found here: https://www.apple.com/legal/internet-services/itunes/us/terms.html
Some awful passages that we had in the last year (i didn't agree with them and hence could not access my purchases) do not appear here anymore, but still, as you may read under the link:
Note this "for instance" in "for instance, because the provider removed it". It is not necessary that the provider (in this case SerifLabs) removes the download. The software may just disappear for whatever reason and there's nothing you can do.
And remember that these terms and conditions have been revised in the past, and hence are more than likely to be coninued to be revised in the future. And every time when that happens, you lose the access to your purchases for as long as you don't agree with the new conditions.