Because if you give the police an inch they'll take a mile. Look at the police in Coventry getting a barbecue to disperse by smashing all their stuff up. I don't recall the emergency legislation giving those powers to the bizzies.
I actually think that this one is a bit more egregious:
GMP Denton/Droylsdon said:Excercise is limited to around an hour per day and not more than once. Please stay home
https://twitter.com/GMPDenton/status/1243502883565428738
Last I checked the regulations didn't set any time limits nor actually, if you read them, a limit on the number of trips out. I have zero issue with the police or anyone else recommending that people do as above, it is after all good advice*, but this is just making up their own rules...
*And personally I'd like to avoid the situation where the government do actually bring in harsher restrictions which will no doubt happen if people continue to take the mick.