Thomson don't issue boarding cards the day before for flights to USA , Mexico and Cuba , although you can check in your bags and get a receipt .
If you look on Thomsons website on FAQS about Day before check in it does state this is their procedure , although they don't say it's because of something thats happened in America ? But flights to America are subject to extra security requirements and Mexico and Cuba are probably included in this too .

Info link. http://www.thomson.co.uk/editorial/faqs/flights/checking-in.html

It must have changed in the last 15 months or so - because we went to Cuba in March 2011 and picked up the boarding cards when we checked in the night before.Seem to remember the "rule" only applied to US and some cruise flights then - suspect the Yanks have made a fuss!!

Don't think (for once) it can be down to the American's. Virgin Atlantic issue boarding passes the night before if you use Twilight check-in.