This only happens to me on Yosemite. I gave Steam access as per my previous question. Steam worked fine until the upgrade to Yosemite. What may have changed, and has anyone a clue how to fix that without deinstalling a few terabytes of video games?
A temporary fix (requires terminal):
sudo sqlite3 /Library/Application\ Support/com.apple.TCC/Tcc.db 'delete from access where client like "%Steam%"'
Allow steam access in accessibility via UI or something like:
sudo sqlite3 /Library/Application\ Support/com.apple.TCC/TCC.db "INSERT OR REPLACE INTO access values ('kTCCServiceAccessibility','com.valvesoftware.steam',0,1,0,NULL);"
This seems to be a known bug for Yosemite.
Steam has to be updated to work 100% with compatibility for Yosemite. Just wait for them to fix it.