From the document that introduces these teams:
"During the 2007, 2008 and 2009 Rules Review, the 5 members of the Blood Bowl Rules Committee (BBRC) (which includes game creator Jervis Johnson) unanimously voted that the following three teams would be endorsed additions to the game of Blood Bowl for LRB 6.0. However Games Workshop will not allow these three teams to be officially printed in the Rulebook until such time in the future when Specialist Games has the ability to create a range of miniatures to properly support all three of their releases at once. So these three teams enjoy the status of being BBRC Endorsed Blood Bowl teams but they just cannot be put in the Living Rulebook yet. In addition, the NAF, (thenaf.net) the official Blood Bowl tournament tracking organization, endorses these 3 teams for tournament play."
In other words, they are just as legal as any of the 21 teams in the Competition rules, and the only reason they're not in the LRB is that Games Workshop refuses to make models for these teams (in particular Slann) or to repackage existing models (Skaven & Goblins for Underworld, Norse/Skaven/Dark Elf/Goblin for Chaos Pact) to be able to sell these teams as teams (the standard '11 or 12 models for£30 0r $/€40' package).
My guess is that GW doesn't want to devote any extra energy and resources to Blood Bowl since, after all, it won't make them as much money as WFB & WH 40k do.
Luckily, there are multiple manufacturers of 'Fantasy Football' miniatures & teams (some of which are advertised on this site), and, being smart, most of those made at least a 'Frogs' team, and plenty of others.