That implies an unintentional error. I doubt very much this decision was unintentional - more likely a business decision based on cost/benefit and/or availability of suitable PSUs.
They absolutely screwed up. They deliberately designed the power switch with a plastic actuator rod, they cut away the main PCB for the rod, they cut away the shielding over the main board to allow the rod to pass through it. They mounted the switch right next to the mains inlet. It is blindingly obvious it was designed mechanically to support a switch switching mains power. If 'soft' power control was an original design intent there would be a push button on the main PCB instead of a cut out for the actuator rod.
Not necessarily - it could just be they were simply keeping their options open. The mechanical and front panel design may well have been fixed long before they had finalised the power solution.
The way they did it gives them maximum flexibility - a lump of plastic is not an expensive part, so making provision for a rear switch,ande deciding later how to use it seems a very pragmatic solution.
If they'd designed a soft switch and then found they couldn't get a PSU with low enough 'off' mode draw within budget,
that would have been a screwup.
Bear in mind that they may have been be planning for an internal battery option, which they could do without changing anything but the PSU. Again the decision on whether they would do this may well have come way after the mechanical design had to be frozen for tooling.
A company that spends millions of bucks developing ASIC-based top-end instruments does not make 'screwups' over something as trivial as a power switch.