can't as in "that asset class does not meet the risk and liquidity requirements for cash management in general"
not can't as in "de-facto illegal."
dereliction of fiduciary duty is illegal, of course the circumstances matter.
I think you are conflating capex + investments with overall cash management.
They already invest a lot in the vision fund. This is a way for them to route more money into the vision fund on top of what they already invest, by tapping into an additional asset class within their portfolio allocation.
If a company's portfolio is a high percentage of cash management products, that's a bad sign of low innovation. If the portfolio is too low a percentage, that's a bad sign also because they would be unable to access liquidity to cover operations given a downturn or sudden need for capex etc... Different companies have different allocations but healthy companies (including google) have allocations into a diverse bucket of asset classes...
> consider this concrete counterexample to your claim
The fact that Google also has a VC arm is not a counter argument. Google allocates some amount of its cash into cash management products as well.
> SoftBank's investors don't need SoftBank to hold cash, treasuries or corporate paper on their behalf
Their customers do. They're still a major telecom company in Japan -- they certainly have operations they need to protect.