1. It is not a scam. It simply shows a complete lack of understanding of how air tickets are priced. Different markets = different prices. You can circumvent the pricing system by making it appear that you are buying in the US. If it were a huge issue for UA, UA could work to end that too, but doesn't bother because few people do it.

2. As to where your seats go if cancelled, they go back into inventory as a seat and are reassigned to a fare bucket by UA's RM/IM software based on the its calculations at that moment. There is no way you or anyone can predict what will happen to your seats if cancelled. This includes whether the seats are sold at all, e.g., the flight is overbooked and UA now wishes to reduce the oversale risk.

It is not a scam and a bit of research will show that pretty much every major international carrier sells at different prices in different markets.