it is a thing not easy to understand with SharePoint deployment. You can deploy to a particular Web Application only if there is something in the package that will add files or modify the files in the Web Application directory especially the web.config file.
If I quickly think, I see a few cases :
1 - the most obvious and frequent: you deploy a component (as a web part) that has to be registered as safe within a particular Web Application. The deployment process will modify the web.config of the Web Application to register the component as safe by
adding a line in the safe controls section of the web.config file of the Web Application.
2 - less frequent: you deploy a dll in the bin directory of the Web Application or add ressources within the directory of a particular Web Application
3 - pretty rare: you deploy a solution that will change the Code Access Security policies for a particular Web Application
For the other solutions, even if you plan to deploy them to a particular Web Application, if there is not anything that modifies something within the Web Application directory, the solution is globally deployed.
So, as in my package there is only a dll and a feature, and because the dll is to be deployed in the Global Assembly Cache, the solution can only be globally deployed. So I think that if you remove the -url parameter in your stsadm instruction, it should
work. (If you had used the SharePoint UI to deploy, you shouldn't have had this problem because the UI wouldn't have let you to choose a particular web Application to deploy...)
Hope that helps...