I think the penalties are in danger of getting in way of the racing in F1, so I think the decision here for Lewis and Vettel was actually a rare sensible outcome.
OK, Lewis didn't get a time or point penalty, but everyone's assuming that means the Stewards/FIA thought Hamilton's driving was completely fine. Maybe they decided, as he's pretty much kept his nose clean so far this year, that a private warning might be sufficient. Maybe they thought it was totally fine or borderline/no worse than previously allowed.
Did I think his speed variation was excessive? Slightly, yes. Did I think it warranted more than a 'calm it down' warning? No. I generally think the penalties in F1 are inconsistent, but I wouldn't have wanted Lewis penalised just for the sake of being consistent with other stupid penalties. I also wouldn't want a penalty just to ensure a closer title battle for the last two races.
The only rule it is possible we have video footage to prove Lewis broke is the five car lengths rule, but this is almost an unmeasurable generality of a rule anyway. Do you mount camera equipment in the exact same location as the spectator was positioned, go out to the track and take measurements to ensure it was never more than 5 cars?
If random spectator footage can be used to prove that any driver broke this rule at any point during a SC period (or any other rule for that matter), and action be taken retroactively, we'd have teams wasting time trawling through all the video footage they could get their hands on. If it went too far, you'd have a team like McLaren employ dozens of camera men to ensure they captured every second of Ferrari's race to ensure they were never outside the rules for the entire race. In fact they'd probably video Ferrari's employees every step in the pit garages too.