Why SQL Server installs multiple services for AS and RS, but only one for IS?
Integration Services is not "Instance aware". The rest (Analysis, Reporting, DB) are.

Note that SSIS doesn't need to be instance aware either. The service itself is pretty pointless and doesn't need to running. You simply need the SQL Server common binaries installed to run packages. The service is for SQL Server Agent and SSMS to browse packages etc

Links:


http://www.ssistalk.com/2009/11/04/ssis-what-does-the-ssis-service-actually-do-anyway
http://support.microsoft.com/kb/942176