It does. The law says you have to implement the OBD spec - and K-line is part of the spec.
The point is that cars have to comply with ODB and bikes do not.
Yes, the law says they have to implement the OBDII protocol. But, there are 5 different physical layers they can choose from:
1. KWP2000 protocol over K-Line, used by most manufacturers, possibly in conjunction with the L line for device addressing.
2. VPW protocol - used by older Ford and General motors and variants thereof.
3. CAN high speed
4. CAN low speed
5. Variants of the CAN but with 29-bit ID instead of 11-bit ID
All of these have to implement the J1979 comms protocol on top of their physical layer. And the J1979 is the mandatory part. However, manufacturers are free to use any of the physical layers... until 2008 that was when Europe deemed CAN a minimum necessity.
So, CAN is a requirement for european vehicles.