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Karl, I had never noticed this behavior. It appears as if you use a parameter inside a calculated field you end up with the original value (and no I don't know a way to change this behavior). But if you use the parameter directly in the viz, you get the alias:
Aliases are not available in calculated fields, with one exception. If you use an alias in a URL Action, Tableau supplies the alias and not the original value - this was true as of Tableau v7, I haven't tested it lately because I went a different route.
If you really need access to the aliases in calculated fields, your best bet is to set up a dimension table in your database and bring that in via a multiple tables connection, custom query, or a data blend.