What? Being "forced to pay" is called being mugged, I don't think you know what scam means.
It's not - "being mugged" is transferring of liquid and portable personal assets in possession from one person to another person usually under threat, or by the use, of physical force with no reciprocation of product or services in return.
Summertime Saga is currently offered by the developer at no charge. A person may decide to donate once or on a recurring basis to show their support of the developer's vision, plan, and goal but are under no obligation to do so in order to utilize the product available.
Since there is a free product offered in exchange for no amount of money - this is not a mugging.
A scam requires the transfer of valuable assets from one person in (
edit: the expectation of) exchange for the receipt something of relatively equal value or consideration, but the other person never intended to honor the agreement of reciprocity.
Again, no one who decides to use Summertime Saga is being scammed because there is no requirement of transferring their valuable assets in the form of fees, duties, etc in order to do so.
Yes, I'm being pedantic because it appears that, for some, this is required.