Tariffs don't need to make foreign oil go away, only be strong enough to level domestic competition a bit. A $40/barrel tariff would still leave very low oil prices for imports, but at least allow for domestic competition... it isn't about totally squeezing a market out, only leveling some unfair trade practices that would hurt consumers/citizens in the long run.
Or maybe there are enough reserves, but they are in the long game and want to buy the foreign oil first... while the taking is good.