Data Bundles

Why are data bundles purchases not fully utilised before switching over to normal data in a cellular contract? For example, if i run out of data by 29 August I buy a data bundle that lasts upto end of following month or 31 days (Freeme) however by 01 Sep any unused data from that bundle is not used, rather the normal contract monthly allocated data is used. This is robbing the customer as he/she would need to use all normal contract data in September to access any remaining data from the bundle purchased in August. This is unacceptable and needs to change!