Why does referencing a variable in a join predicate force nested loops?

I've tried your query on a SQL 2012 instance and trace flag 4199 seems to fix the issue. With it enabled I get a merge join for a total cost of 0.24 and none of the extra branches.

The specific KB article for this issue is Performance issues occur when the join predicate in your query has outer reference columns in SQL Server 2005 or in SQL Server 2008

To further qualify, TF 4199 enables all optimizer fixes. See this link for more information. Enabling everything at once can have weird side-effects, so if you can find a specific fix it might be better to enable the fix on it's own.

You can enable a trace flag on a per query basis using OPTION (QUERYTRACEON 4199);