Yes, since Horizon probably stands to make more money (as a percentage of the retail price) on a Spektrum unit or DSM2 system than on a JR (non-DSM2) one, chances are that they'll effectively block DMSS as an option for JR products -- or they'll charge a massive premium for them.
Of course in these days of online purchasing, you can always buy from overseas -- although the issue of FCC certification becomes a factor then. It would not surprise me if Horizon requires that JR do not FCC certify their DMSS system so as to effectively keep it out of the US market. It's worth noting that the JR XG7, the only radio with DMSS at the moment, has no FCC certification logo anywhere to be seen on it.
RCModelReviews.com, just the facts.