The main reason the Chinese labs are releasing models as open weights is because they don't have the compute necessary to provide all of the inference. For the US frontier models something like 80-90% of the lifetime compute required for the model is inference rather than training. China wants to shepherd as much of their limited compute as possible towards training to keep up in the race.
With nearly everyone using inference accelerators, the pool of hardware is no longer shared between training and use.
No, they are open sourcing them because they don't have another play, being second/3rd tier lans
I think the main reason is to minimize the market for closed-source models from US companies.
China knows that doing what Anthropic/OpenAI/Google/... are doing is impossible for them. No one outside of China in any sane condition will send their data to compute farms IN CHINA like people currently do with US-based frontier models. Even if they could muster the inference power.
Hence they do the second-best thing possible to attack the dominance of the US-based corporations: reduce their moat by open-sourcing models that are not fully equal, but practically useful and good enough for easily 90% of typical tasks people use agents for in their daily lives. But way cheaper to run.
As long as this arms race in AI continues, China as "number two" will have some incentive to continue open-sourcing models. But of course the US government might force a change if they continue to enforce limited public access to new frontier models - there is no market to minimize if a model is not allowed to be publicly available.