Seems like the URL property is not used as a general-purpose field, but its purpose is:
This property may be used to convey a location where a more dynamic rendition of the calendar information can be found.
Please, allow for editing calendar url in case a mistake was made or a calendar url is refactored.
The URL can only be added when it’s correct, so it should not be possible to make a mistake at setup. When the calendar URL is changed, please just add the new one.
What’s your use case?
An url that changed. The maintainer of the calendar decides to change something in the path or hostname. That happens a lot when we are setting up new systems and having to work with test urls that are long and tedious to enter and change only in a minor way.
not quite the solution to the problem, but if you frequently add calendars, you can also create qr-codes (google is you friend, plenty of online generators) that contain the webcal(s)://url.com/here.ics . Scan them on the phone, e.g. with zxing and icsdroid will open them to add a new calendar. That way you don’t always have to type them in.
That is a lot more work than when your colleague tells you, please change the hostname or remove /test from the path. It is changes like that from refactoring or development process that are different each time. If this was to be supported, the cache should be cleared of course.