If a customer of yours pays you to develop a website (not to buy it, so he doesn't receive a copy of the software), the website uses GPL libraries or frameworks, and the site is installed in a server of yours, but it has been done "for the use of your customer", are you somehow forced to deliver a copy of the source code to the customer?
In my case, my website uses a GPL-licensed framework (without the linking exception), which is dynamically linked to the executable that runs the site. The code done by me is not licensed (because it's private, I'm the only one that has a copy of the software and runs it).
At which extent is the app being "used" or even "owned" for your customer in particular if he doesn't have a copy?
The GPL licenses were invented precisely to protect the rights of the user of the software, but was mainly meant for protecting the rights of the user that owns and runs a copy of the software, so they can inspect what is the software doing in his computer and also change its behaviour, because a software makes a user dependent on it.
However, a website is used for everybody with access to the Internet, is not something that users "run", and your customer is a special mention among all of these users, because him is who dictates how must that sofware behave, but has no direct control over it.