@Ben38
For somebody who relies on STR income rather than a job paying PAYE this calendar restriction must have a serious negative impact on your potential earnings in Brighton.
What some hosts don't realise when you list on multiple platforms is that calendars between platforms need to be synched in order not to accidentally double book. This lockdown on your calendar will lockdown your linked calendars too, which is highly undesirable and restricting bookings on other platforms.
I searched Hotel Tonight - an Airbnb owned company and they are offering rooms in Brighton without issue, so i can see no reason why the decision to accept bookings is taken away from hosts on Airbnb?
I do think the choice of whether to have availability should down to every single host as every single host is different and has different circumstances.
Perhaps you can get your dates unblocked by contacting CS. Hopefully you can. Good Luck.