There is no undergraduate degree for law in the US, the degree is only offered on the postgrad level. Secondly, most undergraduate degrees in the US are rigourously academic, yes there is that liberal arts component, but the objective is to give the student a welrounded grounding in various courses, usually in his first or freshman year. After completing this grounding, the student then spends the next two or three years specialising in his chosen field of study.
Unlike at UK institutions, students have classes in each subject 2 -3 times a week, whereas over here it is once a week plus tutorial. Therefore there is no lack of academic rigour but perhaps a more condensed style.
Also what most UK unis cover in a year, depending on the type of class and the amount of class time, the US unis may cover in a semester (half a year).
IMHO neither is better or more rigourous, they are just different. In the US there seems to be a continuous assesment style, while in the UK, there is the one off type of assessment, at the end of the year... you only get one bite of the apple.