I emailed Forgeworld regarding this exact topic. Their response was they have done this deliberately to prevent spamming Relic LoW's with Super-Heavy Auxiliary Detachments. They conceded that they have made fielding these models difficult and are going to look at possibly adjusting things in the future. The only solution now is to use a Super-Heavy Detachment. I suppose the easiest way is running one Relic LoW and two Knights or throw Guilliman in the mix. I'm really glad I didn't purchase the Spartans that were on my list!