I noticed that they didn't ban "walkie talkies" or "radio transceivers", and I wonder...

Some of the more niche/hobby transceivers could be used by a group of bad actors at an event, for comms that are less-monitorable than smartphones and mainstream COTS handheld transceivers.

And such hobby transceivers/transmitters might not operate on the list of RF bands that would be jammed by authorities when there's a suspected terrorist situation.

Many of these devices have exposed PCBs (either general-purpose SBCs, or specialized). So, saying "no Raspberry Pi" could be an attempt to ban all exposed PCB devices. And "no Flipper Zero" is the non-exposed-PCB problematic device that everyone has also heard of.

Kudos to the people keeping the event safe, especially given all the recently emboldened bad actors right now, who might be attracted to the event.

For hobbyists, there are numerous opportunities to advocate for your right to, e.g., carry electronics hobbyist gadgets, or to wear an artistic blinking LED jewelry piece strapped to your chest. Some of those opportunities need help, while some other opportunities could be counterproductive to your cause.