The following image is my question. SO would not allow me to post because of some error about formatting code incorrectly that I couldn't figure out how to resolve after an hour of trying.
I've been bitten by this too. It boils down to limitations on CloudFormation's intrinsic functions (source):
Note
You can use intrinsic functions only in specific parts of a template. Currently, you can use intrinsic functions in resource properties, metadata attributes, and update policy attributes.
In this case, "resource properties" only applies to the contents of the Properties field. I would expect this to change, since I suspect Amazon has gotten many complaints about this! I'll happily update the answer when that happens.
