I think both 'admitted stealing' and 'admitted to stealing' are acceptable, but 'admitted' and 'stealing' being separated by another word makes the 'to' obligatory.

Actually I find the fact that there's no 'on' with 'Thursday' and the confusion between 'by' and 'buy' far more disturbing.

The version without the 'on' is common USn journalistic style, the 'by' for 'buy' is a perfect example of the inadequacy of spell-checkers.