It's perfectly simple. If the terms agreed with the EU have an impact on the ability to deal with other countries, for example by allowing for regulations made in the EU to apply in the UK, then Britain will remain restricted by those terms. Deals are not done in isolation.
Jeremy
Thanks for clearing it up.
So Trump's comment to May, "You can do what you want." is his unhappy acknowledgement that GB will do and arrange Brexit with the EU however they need too. Prior to that,Trump was trying to pull GB away to negotiate with us directly. It would have been a wedge against the rest of the EU. It would also help us with tariffs with other non-EU countries like China especially as it would be the first crack in the tariff wall. I guess May told him that it would probably be impossible for GB to make that deal with the EU during Brexit because Britain can't afford not to be able to trade with the EU countries.
Of course, if they take on the same tariffs as the EU, they will be treated the same way as the EU. So US tariff would be applied to BRitish products as well as the EU's. So he will negotiate with the EU primarily at that point. If by chance, GB does not have to follow the EU with tariffs, then we will negotiate a direct deal with Britain. The EU will follow Britain.