They get around this by limiting the amount of payment to the drivers' costs (i.e. the IRS standard mileage rate of $0.54/mile.)

From the TOS:

"...we may charge you to reimburse the Carpool Driver for the costs associated with a Carpool Ride provided to you. [...] Reimbursements may include per mile charges at up to the prevailing IRS standard mileage rate, and other expenses like tolls and parking fees." (Emphasis mine)

I think it's debatable, legally, whether or not this constitutes commercial use. It's designed only for picking people up along a route you already drive regularly, and the monthly cap of $400 means no one could conceivably make a living off of it, even if they gamed it somehow and had multiple accounts.

The amounts seem purposely low so as to appear like non-commercial activity.

Pretty sure normal insurance covers giving rides so long as you don't profit -- that is, as long as your compensation is no more than the cost of giving the ride, at which point it would become commercial activity, with a different risk profile.