Universal doesn't profit off of Youtube clips so the IP-owners don't have much to say in the way of legality on the issue. A DVD for sale is a different story, though, since then Universal is making money off of someone else's property. The thing about these HHN contracts is that they are written tightly. The owners of Halloween didn't just say "here's the movie, do with it what you will"...it's more like "you can use these specific characters in this context, use these settings, use this music in this environment but not this environment, have this type of merch for this amount of time, have our logo take up x% space of advertising", etc. Everything they can and can't do is outlined specifically and there isn't wiggle room. So unless an IP-owner says "you may use our characters in a DVD" then Universal can't do much. And since no such DVD exists or has ever existed, there's no reason to specify such a thing in a contract--therefore no DVD release.
Sorry for the drawn-out post but I've always thought copyright laws were pretty interesting, but a lottt goes into how this stuff works.