The moto into which Porte crashed was itself held up by two or three other motorbikes in front of it. WTF do they need so many of them on the race for?
...and the motos in front of it appeared to be blocked by the crowds, so the outcome might have been the same had there only been one; and as far as I know we don't know if there were cars stopped in front of the motos?
I don't think the root cause was motos here, but I do wonder about moto behaviour; the one behind Froome was clearly riding too close as it wasn't able to stop in time, and I wonder whether the one in front of Porte had to stop so hard because they were too close to each other - each having to brake progressively harder until the last really had to stand on the brakes.
One thing that did happen, because we saw it, is that after the accident the road was partly blocked by motos either because they weren't able to get out of the way or because they were all anxious to see / get the best shot of the crash.
Truth is it's a complex situation with no simple answer, but the number of motos might not be relevant had the crowd control been adequate and the available road been wide enough. Given the need to get cars as well as motos up the stage and enable attacks, it seems to me the organisers should be able to guarantee a road width of, say, 6' 6" by barriers or other means. I guess they've shied away from it in the past, as
@Crackle suggests, because of the difficulty of erecting extensive sections of barrier in the limited time available (rather than just pure cost).
There is, in the UCI regs and in the regs issued for individual races, quite an extensive section about the control of the official convoy, behaviour of press and other vehicles etc., with diagrams of the convoy layout and so on, which might clarify questions about the number and use of motos. Unfortunately I don't think ASO are as good about making this information as easy to find as other organisers are, but a google might turn it up.
Good point that wasn't mentioned on tv.
Increased crowds mentioned several times upthread, and also discussed by Imlach and Boardman, in the live ITV4 programme at least. The big difference between this stage and 'normal' mountain stages to me seemed that while the big mountain stages are always (stupidly) crowded, in the section where the crash happened the crowds were so thick that those out in the middle of the road weren't able to retreat back out of the way, and the idiots running were actually running among the bikes and motos rather than alongside - and alongside is risky enough.
Does anyone think that there was a concerted (and planned) effort by the spectators to deliberately try to stop Froome when they saw the yellow jersey - remember last year with the urine etc.
No. In that chaotic crowd? Good luck trying to organise that lot to do something concerted. Not even sure that where the motos stopped, the crowd in the road could have seen it was Froome approaching.
Could drones be the answer tv coverage wise?
Seems to me there are too many negatives to drones at the moment; reliability, battery life, risk of danger to the public, aviation regulations, limits of control (line of sight, distance, keeping the operator close enough to the drone given the crowds, obstructions to radio transmissions, windy conditions and unusual air currents / eddies round mountains, etc.) and so on. Given the speed with which drones are being adopted for other work I'm sure it'll come, but I'm not sure it'll come quickly and in any case, drones wouldn't replace much of the official convoy and wouldn't have resolved the crowd control issue that was the issue here and seems to be a fundamental problem on the big mountain stages.
It'll probably be CHF200 fine for their team but there seems to be some delay publishing the jury decisions today 
