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.