BYOD does NOT mean use insecure services
Wait a second here - proper corporate use of BYOD is via secure client apps which never store corporate data on unauthorised external entities and allow the user to connect to internal data storage and services such as corporate email using secure communications.
It is common sense to discourage or prohibit employees to store company data on personal cloud space, send business emails to personal webmail account, or pass company information through external services.
This is not new - staff of large companies have been told not to use Google translate for business purposes for years!
This has nothing to do with BYOD, its about having a proper security culture and the right tools in place for staff to be able to work full within the boundaries of the companies systems.
BYOD is a healthy revolution, but you have to do it professionally and right!