I boarded, the service as usual in business was fantastic. But, due to the fact that I hadn't paid for the ticket I only accepted the most basic of services. I didn't use the free amenities pack, pyjama's or blanket. I had the meals which are always good on Qatar whichever class, but mainly had water, rather than the wines.
Why not? The airline made a business decision to fill up all the seats on the aircraft. They probably had already loaded all the frills for the occupant of that seat. If nothing else it comes from their marketing budget, you get a good experience and they hope that in the future you'll fly with them again and possibly pay for more expensive seats. If not, then as a regular customer, they're saying thank you for your continued custom.
If the airline chooses to give you a free upgrade then enjoy it, you might never get another chance.