My take: Initially early digital foundries simply followed Adobe's model, where modification was explicitly allowed in their FAQs (even if their EULAs were more ambiguous).
Adobe took this approach because they just didn't want to have to deal with the hassle of one-off modifications and licensing requests - they could leave this in the hands of customers - "modify the font as you see fit but make sure whoever you give it to has a license to the original".
I think most type designers today want to be involved in modification - either doing that work or approving of the work - this extends to open source fonts too - the OFL restrictions around font names being evidence of this.
I wonder if promoting the font as open source is misleading. Why not say the font is free to use and modify under the SIL, and that the intent is to open source it within the next 13 months? Alternately set a lower bar for the sources, and publish the Glyphs sources.