I wouldn't be so hard on them. The sticker said 3000, so they were trying to be nice and get it done. Although they really shouldn't do anything without you knowing. Sounds like they tried to make it right with putting in the synthetic and giving you a coupon. Think of it as a free oil flush
Just be more explicit with them when you drop your car off. Tell them exactly what you want done (I want synthetic 5w30) and to speak with you before they do anything else. It can be frustrating, but I think they had your best interests in mind, they just were bad about catering to your specific needs.
p.s. the first thing I do after an oil change is remove that sticker then login to mysubaru.com