The user (of the proprietary software) is forced to execute on his machine whatever computation that software tells him to. He has no way of knowing what that computation does or modify it. In this case, developer holds unjust power over the user because the developer can make the software do whatever he wants, and even change the behavior of the program by auto-updating it over the network - and the user is completely powerless against the developer.

Yes, he can choose not to be a user (of the proprietary software). That does not make proprietary software any less immoral. Being able to refuse to be a part of a pyramid scheme doesn't make pyramid schemes any less immoral.