Before I got a job at a dedicated bike shop I worked at Dick's Sporting Goods in the bike department. If you're looking to ride occasionally on fire roads or improved dirt paths, those bikes will suit you fine. However, if you are looking for a bike that you can ride aggressively repeatedly I would look elsewhere, say, an actual bike shop.
Another problem with buying a bike at Dick's is that most of the people they have build the bikes are not actual certified mechanics. I spent more time fixing bikes that the other "bike techs" had built than actually building bikes. Also, when you buy a bike from a shop, you are also investing in that particular shop. Say you buy a bike from Dick's and a comparably spec'd bike from an established bike shop. If the Dick's one breaks it'll take at least 3 weeks for them to get the parts, and at least another week for someone to actually fix it. At the shop, they'll most likely have the part in stock, and you'll have years of experience working for you.
The short answer is, those are "disposable" bikes. It'd be easier to replace it if you do break it, and they are not designed to hold up to downhill riding
