The Go developer goes to a lawyer and asks for advice. The lawyer's advice doesn't have value for the lawyer. The law the lawyer uses isn't better. This is also why designers are rarely interested in designing for open source projects.
In terms of generosity, as theorized the Go developer's bug fix benefits the Go developer. It likely also benefits Google, a subset of six-figure Silicon Valley engineers, and a few Sand Hill Road limited partners.
In the US lawyers "give back" via pro bono work. The beneficiaries are not corporations, investors, and well paid professionals. When lawyers give back, they often work on the intractable problems of poor people. Pro Bono work is pro bono because it is clearly not in the lawyer's basic business and economic interests.
A lawyer's free legal advice is required by law to meet the same standards as the lawyers paid legal advice. The free tier has to be just as legally sound as the paid tier. This extends to online forums.
Besides, if a person is able but unwilling to pay for legal advice, their problem isn't very serious.