Because we don't have a right to a continuing service that requires their labor unless they agreed to it. A buyer should discount the value of products that rely on ongoing services accordingly.
See also 'Juicero'.
> Because we don't have a right to a continuing service that requires their labor unless they agreed to it
There are carve-outs in the legislation for this. It's a moot point.
If a software requires a server component that is costly to run, then I would expect the software developer to charge a subscription in order to use it, rather than offering it as a one-time charge and then destroying it when they realize letting me continue to use it is costly.