I agree with most of Karena's comments in this interesting article. At Xafinity, we also treat SIPP and SSAS investments the same. SIPP providers have more exposure to potential tax charges, especially the 15% sanction charge and are therefore providers more likely to be cautious with investments.
The problem with "regulating" the SSAS market to align it with SIPPs is that 10 years ago, HMRC introduced rules that allows any Butcher, Baker or Candlestickmaker to be a SSAS Administrator. Unless there is a return to compulsory "SSAS professional trustee", then regulation will simply not work.