Having only been around 2 GM events I can't attest to crowds of non-RPers, both events I witnessed, although there were more people about than usual, they were acting quite IC for the situation. (Can't attest to how IC they were for their normal RP, since I didn't know the characters... but they were playing the situation as I would expect.) My character is the type who's likely to investigate any crowd or commotion just out of curiosity, so it's easy to fit in. I certainly wasn't there for the reward, since I don't recall receiving any... But I suspect that without the label, neither event would have generated much action.
But I am puzzled by some of the responses about events affecting roleplay. Hydlaa is a city. Presumably, this city has more inhabitants than the handful actually visible wandering around at any given moment. In RealLife, I see spontaneous crowds form on a regular basis. Why? I don't know. GM event? Probably not. Free beer? One can hope. Britney Spears? I try to avoid papparazzi events. Regardless, the point is, it doesn't matter. The crowd formed, and no one asked me ahead of time what I thought about it. Did it ruin my life? No. I adapted that minor period of my life to deal with the unexpected event and moved on.
Roleplay should be equally flexible. Especially when the world you are RPing has magic. So you and your friends were sitting in Kada-El's when a crowd showed up and ruined your roleplay. Strangely enough, I find that taverns are crowd magnets. When I'm in a tavern, I EXPECT that to happen. Does it matter if its a GM event with a bunch of GM groupies running around, or a wedding party with a bunch of drunk obnoxious underaged second cousins running around. No, the end result is identical. Is it your tavern? No. The bartender sure isn't objecting! So if your roleplay is so inflexible as to be "ruined" by a crowd showing up, in a setting where a crowd is certainly not unexpected, I'd say that is a problem with your roleplay, not the crowd.