I have no idea what the criteria is for requiring a 3-prong grounded plug in consumer goods.

In the UK at least (and I suspect these things are international accords), the criterion for a 2-prong mains connection being acceptable is being double-insulated: in other words, made so that no single breakdown of an insulator can expose mains; two coincidental breakdowns must be required. That's why mains cables run each conductor in its own plastic sheath, and also have an overall plastic sheath. If either one fails, mains is not exposed; both must fail coincidentally.

The symbol for double-insulated is a square box inside another square box. You can see the symbol next to the (two-pin) mains feeds on Rio Receiver and Rio Central.

Anything that's not double-insulated, must be grounded (i.e. have 3-prong mains).

Peter