Except they weren't. US crude producers were getting LESS than the spot price on the global market. Because they couldn't export. This is what naturally happens when domestic production exceeds domestic consumption, but the surplus can't be sold abroad. Which was the intention of the law when it was introduced.
You would be correct if domestic consumption was still greater than domestic production, the domestic price would still be tied to the global price. But that hasn't been the case since the May of 2011, when the US became a net exporter, until December of last year, when the crude export ban was lifted.
"Prior to September 2010, there existed a typical price difference per barrel of between ±3 USD/bbl compared to WTI and OPEC Basket; however, since the autumn of 2010 Brent has been priced much higher than WTI, reaching a difference of more than $11 a barrel by the end of February 2011 (WTI: 104 USD/bbl, LCO: 116 USD/bbl). In February 2011 the divergence reached $16 during a supply glut, record stockpiles, at Cushing, Oklahoma before peaking at above $23 in August 2012. It has since (September 2012) decreased significantly to around $18 after refinery maintenance settled down and supply issues eased slightly."