This could be Google's motivation (although note that Google is not actually the market leader right now) but the risk could still be real. Most of the signatories are academics, for one thing, including two who won Turing awards for ML work and another who is the co-author of the standard AI textbook (at least when I was in school).

You can be cynical about corporate motives and still worried. I personally am worried about AI partly because I am very cynical about how corporations will use it, and I don't really want my atoms to be ground up to add storage bits for the number that once represented Microsoft's market cap or whatever.

But even cynicism doesn't seem to me to give much reason to worry about regulation of "next to no cost" open source models, though. There's only any chance of regulation being practical if models stay very expensive to make, requiring specialized hardware with a supply chain chokepoint. If personal devices do catch up to the state of the art, then for better or worse regulation is not going to prevent people from using them.