I doubt they did this to avoid the passport control stamp issue. You're one passenger amongst thousands needing a stamp every day at AMS
Most likely reason (though I've not checked the actual timings) is the LH routing was to get you to SFO without triggering EU261 Compensation or because KL to LHR would have exceeded the LHR passemger cap
Also it's VS that flies to SFO from LHR not DL and VS could well have been full or restricted because of the cap. DL would require a change somewhere and again cause a delay ion getting back to SFO and triggering EU261 compensation.