Ebates cash back on gift cards is not a guarantee. It should be treated as an exception to get the cash back, rather than considering it a given. Adding a filler item increases your probability significantly, but it does not make cash back inevitable.
It looks like $200 worth of your gift cards was recognized as being gift cards and flagged to not receive cash back. Just be glad you got what you did - don't open a ticket with Ebates as that may result in them realizing cash back should not have been credited for the additional $100 as well.
It's hit or miss. You hit and missed with your order.