You spent money on a ticket that you then skipped.
You received a voucher for this, purchased another ticket, and skipped it again.
All tickets were for you. The difference for Delta (or any airline in this case) is they get $350- $400 cash for your +1's ticket. That's the real financial difference for them; they're running a business after all.
Everyone has family issues and career issues that prohibit flights. Delta isn't trying to ruin your trips; but as the poster above stated, your second use of the voucher attached it solely to you.
Your threat to go elsewhere doesn't mean anything to Delta because other airlines have the exact same rule and Delta is one of the most generous airlines at making exceptions and bending rules in cases like this.