You are /effectively/ able to install applications anywhere you wish, but this is done using symbolic links. The 'real' copies are stashed away in a fixed heirarchy, by default rooted in your Apps directory.
This seemed like a good compromise between the needs of the package manager (which benefits from knowing where the files are and having control over part of the filesystem) and the needs of the user (who will often wish to put applications and data close together).
An additional benefit is that you can have multiple copies of the application if you wish. (For example, you may have several users who each want a copy of something in their own working directory.)
RiscPkg will probably provide an easy mechanism for creating these links at some point in the future, once I have worked out a suitable user interface and settled on which type of symbolic link to use.