ReportBuilder / Report Model Explanation Needed

(I wish I was able to attach pictures....anyhow I'll try my best without them)

The customer's goal is to via ReportBuilder to select fields from multiple
many-to-many relationships. They are able to see the fields that exists
within the various many-to-many relationships but IS ONLY able to select
fields from ONE many-to-many relationship. Example using AdventureWorks:

DataSourceView (relationships automatically generated by wizard)
Contact ---< VendorContact >--- Vendor
Contact ---< StoreContact >--- Store
Contact ---< ContactCreditCard >--- CreditCard

So the goal in the ReportBuilder is to be able select fields from
Contact/Vendor/Store/ and CreditCard.
PROBLEM is that the ReportBuilder ONLY allows user to select fields from
Contact/Vendor OR Contact/Store OR Contact/CreditCard (exclusively)

The only workaround I've found is to reverse the relationships in the
DataSourceView to the following

Contact >--- VendorContact ---< Vendor
Contact >--- StoreContact ---< Store
Contact >--- ContactCreditCard ---< CreditCard

This works however we need an explanation as to why it only works this way?

Thanks in advance