It ain't just Rasho. If you looked a FT% of players before, during, and after they came to the Spurs, one might be forced to conclude that the Spurs somehow actively teach poor FT shooting:
Speedy Claxton:
01-02 PHI .838
02-03 SAS .684
03-04GSW .813
04-05 GSW .810
Robert Horry
01-02 LAL .783
02-03 LAL .769
03-04 SAS .645
04-05 SAS .647
Jason Hart
03-04 SAS .767
04-05 CHA .862
Hidayet Turkoglu
02-03 SAC .800
03-04 SAS .708
04-05 ORL .839
Only Steve Smith and (so far) Brent Barry in recent year have been able to avoid the Curse of Spurs' Charity. (Danny Ferry didn't shoot that badly, but still had his two worst season FT% of his career in his three seasons with the Spurs.)