The more parts you add to a bike the more it has to cost or the lower the quality becomes. In the case of bikes like you and others have described the quality is at the absolute lowest end you could get. If they had built a fully rigid single speed at that price they might have had something a little better in quality but still would be pretty low. Big Box stores don't have actual trained mechanics that are properly assembling and tuning bikes. They are getting paid by the bike so if it is unsafe and the parts are unsuitable they don't care as long as it is put together enough to look like a bike.

If you need a full suspension bike, save up a lot of money otherwise don't waste the time and money.