It's a great show, though some episodes are better than others. I do wish they'd mind the outbursts a bit more. Yeah, they do bleep out the profanities, but come on, folks. A lot of these games are family games, and this is an internet show with wide age appeal. Consider the broadness of your potential audience, and temper your behavior accordingly. You can have fun without shouting obscenities on camera, even if you know it's gonna get -ed. A modicum of maturity and consideration is called for in any broadly public venue.
YEah sponsors would like positive reviews. Puts you in a tough spot if their wonderful new product is craptacular.
I write on occasion for a book review journal. That journal is funded by sponsored advertisements by publishers. The journal has a policy of publishing no negative reviews. However, that is not the same as publishing false or misleading positive reviews. Instead, the policy is quite simply "if the reviewer doesn't like the book, we don't review the book." That's all. Keeps everyone happy, is completely ethical and above board, and is no disservice to the readers.
Table Top could easily have a "right of refusal" clause in any sponsor contract, which is simply that "if we don't have fun playing your game, we don't cover your game." Which is pretty much what they already do. A wise sponsor will accept that, and accept that the burden is on the publisher to produce an enjoyable game.
So I really don't see the danger of a bad product being a risk to Table Top or the sponsor.
I suspect the real issue is that a direct sponsor would expect its games to be preferred for episodes, potentially to the exclusion of competitor's products, especially in cases where products may have similar themes (as, say, Zombies!!!, Zombicide, Last Night On Earth, etc..). While it's great if Table Top depicts your game as being fun, what if it also depicts your competitor's game as being more fun than yours (even if not directly stated on the show)? That's a sticky wicket for both sponsor and Table Top. Wheaton clearly wants to promote games he likes, and that may not be conducive to a sponsorship approach.