Why do people use "Single-Counted" turnover ratio when modeling for transaction cost. I read a paper which uses only the purchase side as turnover measure multiplied by a spread assumption.

This seems to assume that the sell side does not cost anything.

Paper:
http://www.efmaefm.org/0EFMAMEETINGS/EFMA%20ANNUAL%20MEETINGS/2015-Amsterdam/papers/EFMA2015_0283_fullpaper.pdf

Thanks & Regards
James