Shouldn't taxis turn off their engines when stopped?

Is there any reason that whenever I’m riding a cab and go to the convenience store and the taxi driver (supposedly) knows he’s going to wait for a while but he keeps the engine running?

Wondering if there is any plausible justification or if I can confidently tell the drivers to turn off their engines while they’re waiting for me.

Because they always keep it running and it confuses me.

Thanks