Why did the british start clamping down on American shipping in 1805?

It was actually 1807 when the British began clamping down on American shipping:

Orders in Council (1807)

- In retaliation for the Napoleonic Decree, Great Britain issued a series of Orders in Council that severely restricted American trade with Europe - specifically with France and its allies.

- The Orders allowed the the Royal Navy to board and seize American ships suspected of trading with the French.