I am aware of that most favored nation clause, however here we are talking about a completely different model, called HD-only, so far even the currently "HD-only" pack is not so, because they carry both the SD and HD versions. That is likely because Charlie had not negotiated a new arrangement on those channels, so the existing ones applied, if you have the SD one, you get the HD one.
And Charlie is making money NOW with this $30 pack as you stated there are very few to begin with, so the fees paid to the producers can justify the price tag.
But if they are to add those other 25 new HD channels, the cost of all the HD's will add up, and if Charlie can not increase the $30 price, he will suffer.
I think he is trying to convince the new HD providers his vision of a true HD-only idea is the way to go, and the content providers should embrace it and agree to charge less so he will just carry the HD version, not the SD version. In such arrangement the most favored nation clause may not apply, or if so the table will be turned.