LabelWorx has adopted the policy of not allowing web addresses on release cover artwork because Apple has imposed this policy for their iTunes store.
It is easier for LabelWorx to have you amend a release artwork that contains a web address before we approve your release for distribution than trying to amend the artwork after we have already started delivering it because iTunes has rejected it.
It is believed that Apple has this policy in place because web addresses on artwork can lead to the advertising for rival services to iTunes.