Nothing is for free, but the consumer rarely ever thinks about it being built into the price and instead looks at the excluded bag as just another thing they have to buy. Instead of up-selling, which a lot of people hate and only see it as a money grab (because it is), why not include it as a nice gesture even if they are paying for it? The psychological aspect that they are getting something for "free" goes a long ways in buyer satisfaction.
The $100 "discount" is a sales strategy to make it come in at a lower price point and look more attractive. The bag did not cost $100 to mass produce. Instead of including it at a lower price point (for example, it may have cost $25 to make, but you include it at $50), you're now making more profit by selling it separately at $100. It's business and I can't fault them for making as much money as they please, but as a consumer who uses one of those amplifiers from another company that included a bag, I kindly appreciate their own marketing strategy and making sure it was included - cost passed on to me or not. Sometimes that goes a long way for brand loyalty knowing that someone at least thought about it or listened to consumer requests.
