"But they do not need them."
Translation:Oni je ale nepotřebují.
It is the first unstressed position in a clause. It follows the first phrase which is stressed. Here the "je" is in the second position. There may be multiple clitics placed in the second position at the same time in certain order.
See the Tips and notes and https://en.wikipedia.org/wiki/Czech_word_order#Clitics for more.