Let's Play videos (with commentary) and No Commentary Walkthroughs (that don't have commentary, obviously) are both a legal gray area. There haven't been any court cases, so there is no precedent on what is fair use or not. However, it's probably not likely that either is fair use (even if Let's Players want to think that their commentary is "transformative")
So, instead, the reason Let's Plays and No-Commentary Walkthroughs stay up is more because most game developers (but NOT all) simply never pursue any action.
You have to look up what each developer's policies are. Some are OK with gameplay as long as there are no spoilers, no pre-release material, and no cutscenes. If you search for "Let's play friendly developers" you should find an article collecting several developers' responses...these should generally apply to no-commentary gameplay.