The Petronas bike was right up the line and partially over it on the track side the whole way up the straight from turn 2: if you watch the Moto 3 race, you'll see them over that kerb whilst racing so the fence needed to catch Franco's bike would have nailed a few of them. Bit harsh a penalty for being nudged over the line.
Catch fences were done away with on tracks long ago because they were a bigger danger. You end up with posts flailing around to impale people and an entangled rider to extricate from netting strong enough to catch a 200mph bike before they expire.
Moto GP have said they'll extend the air fence and that's something the riders don't seem to want:
"In response to this, MotoGP will extend the air fence on the outside of Turn 3 to create a block for the entire corner in a bid to avoid any potential repeat of last Sunday’s terrifying accident.
This was an idea put to some of the riders post-race on Sunday by the media, with LCR’s Cal Crutchlow highlighting the risks of extending the air fence.
"If you put a catch fence there, if you extend that catch fence, what happens if you run off track? The place is not safe," Crutchlow said last Sunday.
"Unfortunately, there's no way around [the Turn 3 problem] because the riders would say [if] you extend that there and you make a mistake... imagine in the wet?
"Just running off the track there or something like that and you're alone in the practice, you can potentially go across the grass, go straight, whatever.
"But if you put something there you ride straight into it. We're in a no-win situation unfortunately in regards to that.
"I'm shocked at seeing it, I'm shocked at what happened. And I don't know what they can do."
If there was something easy that could be done, it would have happened after Sato lost it going into that corner in 2002 and hit Heidfeld square on. The gravel traps are extended but that's about all they could do clearly.