Greg, Thanks (again) for your reply. I tried your experiment and it indeed shows that when using an InfoPath developed data source, the "Customize Commands..." button is NOT disabled, but when using a Web Service as the data source it IS disabled.
I don't know why this is, but I discovered a work-around to prevent users from removing rows from the repeating table, which is to manually adjust the manifest.xsf file. In my case it involved commenting out the following line in this file:
<xsf:button action="xCollection::remove" xmlToEdit="Benchmark_7" caption="Remove Benchmark" showIf="immediate"></xsf:button>
This solution is less than ideal because the xsf file gets updated each time the template is updated via InfoPath. But it works.