This will have interesting impacts on all marketplaces. Ebay, Walmart, NewEgg, all the app stores on every platform - if a game is malware (or trashes your machine with a bug), is that Steam's responsibility? It's delivered from their servers with their packaging.
Depends on how much effort is shown to have been made by these app stores to prevent malware. For example, if Steam shows significant cases where malware games have been removed, and can prove they actively work to remove these cases, then it won't be much of a problem for Steam. They are doin their best.
Strict liability means even if they do all of those things they are still liable.