it kinda sucks but I have to agree. Sometimes you just have to look at it from their perspective. Its easy to say oh but its free advertising. but sometimes, like you said, this free advertising can make you lose some sales.
For example, sometimes some people are curious, so they check out an hour or so of gameplay. But because of this, some will decide not buy the game anymore because they think theyve seen enough of what the game has to offer. Or perhaps, you get immersed into the video/game only to find yourself already watching the first half of what the game has to offer. At that point, many would just rather watch the 2nd half. If this was avoided, who knows just how much more sales this could of become.
That said, there are companies who still do something to make streaming and uploading content possible like Nintendo, yet we still hear people complaining just because they dont want to share a part of their profit. However these people are acting as if Nintendo is absolutely disallowing you from using their content when its not the case. Reason I added this bit is just to reinforce the fact that some people always want things their way, but in reality, just because you cant have it your way doesnt mean it isnt fair. Like your last line, these people need a reality check.