Bollocks. Intrinsically, it is not. The only argument here is that such-and-such OEM doesn't want to stock two kinds of DIMMS for their products. The retail side shows no indication that 2x4Gb is any more expensive than 1x8Gb. Maybe it's logistically more expensive to keep 4 Gb SODIMMs on-hand, especially if/when the market grows beyond 8 Gb lappies. Said OEMs don't want to be left holding the bag.
Otherwise, dual channel isn't more expensive.
Most OEMs care nothing about the upgradability of their ultra-thin, ultra-cheap laptops/2-in-1s
The TDP/thermal design spec of the platform was drafted with full use of the IMC in dual-channel mode in minds, so it's generally impossible to argue that any OEM is going to go for single-channel because "it's too hard to cool a dual-channel configuration". Unless they're going off-spec. Which they might be.
Let's face it, single-channel RAM options are beyond ridiculous for AMD or Intel laptops. If AMD can make Raven Ridge work well enough that the end-user doesn't notice or care about the RAM screw-up then fine, good for AMD.