Because my client might find out and use your service for free
We have taken great lengths to remove all Pagelime signatures from the system. You can completely white-label and brand the CMS. We leave nothing in the markup, we leave nothing in the backend, and we have even gone so far as to have "Client CMS" come up as the charge on your clients credit card statements.
However, even if your client knows you use Pagelime, they also know that you built the site and you implemented Pagelime. You should explain to them, that Pagelime is great, but it’s merely a tool for managing their content. By subscribing to your service/instance of Pagelime, they are not just getting a great tool but technical support and the value of all your experience. Additionally, some of our users include free adjustments of editable regions while in a subscription. One of our users has a great method of feeling out clients and providing good motivation to use Pagelime, you can read about it on our forum, here.