Because otherwise you can't do anything with the documents except from the one computer that you put your $400 software on.
And the licensing model obviously is completely orthogonal to the execution environment.
(The realistic answer is "can't unless I brought my laptop" AFAIK.)
Other solutions probably would be a different licensing model or an open data format, possibly converting your InDesign document to an open format beforehand, or logging into your computer at home from someone else's machine, or in some other way using software on your machine at home remotely, or probably numerous others, most of them probably with a huge advantage in terms of privacy/resistance to surveillance over a solution where you process your own data in unencrypted form on other people's machines.
And, actually, the way the "cloud" was understood before the hype, your documents are "in the cloud" as soon as they are on a computer that is connected to the internet. I don't need a "cloud provider" or even a "personal cloud" in order to access the documents stored on my machine at home from my laptop, both are connected to the internet, that's all that is needed.
That it might be possible to make all of that easier to use is still true, though.