I’ve just found this question in a sysml discussion forum:
Is it really not possible to place requirements objects on an Activity, State Machine, or several other similar behavioral diagrams?
Yes, it is not allowed to put requirement elements on a state machine or activity diagram. Since it is essential to show such cross cutting relationships SysML provides the callout notation. A comment symbol is attached to the state or action or to any other element in the state machine/activity diagram. The text inside the comment describes the relationships to the requirements. Although it looks like a simple comment it is a formal representation of the model. A good SysML tool could display the callout notation automatically.
The callout notation works for the allocate and the requirements relationships.