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It shows a certain attitude to cycling - that it is seen as a sport rather than transport, hence an activity to be limited.
And I think it is more about wanting to prevent a loophole in the lockdown restrictions which might make it harder to enforce them as a whole, rather than responding to actual risk.
In contrast, the advice from the German and Dutch governments is that people should cycle.
Also, the Italians, Spanish and French have not banned other activities such as home DIY, which has a much higher rate of injury than cycling, they have not reduced speed limits for motors, etc.
Recreational cycling is banned altogether in Spain!