I'm not sure about the terms and conditions of the Twitch. Based on that it is quite an easy thing for the Twitch team to stop users from creating and broadcasting their contents. For now gambling is getting popular around and Twitch got importance for its scheduled streaming and different giveaways. As one of the user stated, people will move towards other forms of streaming opportunities.
I think everything depends on the TOS of Twitch, like you mentioned. If they prohibit any form of solicitation whether express or implied, then they can rule that this Twitch streamer violated their TOS. Given the widespread of online gambling among the youth, I do expect that Twitch will soon amend their TOS or implement it more strictly this time.
It is really a fine line between a violation or a loophole. I hope that Twitch creates or releases a statement from prohibiting "express" forms of gambling since they cannot totally remove and prohibit crate unboxing from their platform.
This matter will highly depend on what kind of rules and policies does Twitch have on their platform. If they impose strictly that no solicitation should be done in their platforms by the streamers to avoid abuse and scamming, then it is clear as day that Sliker violated their TOS and therefore deserves a punishment so that the others won't follow his steps. However, it isn't outright stated that Twitch is against such act, so perhaps that's the reason why there are still streamers that continuously to do such act.
Since like you said, their TOS isn't clearly implementing what needs to be done, they somehow play a part and have a shortcoming on this issue. Hopefully, they will implement strictly their policies and make those people who won't abide to be accountable to their actions.