Skipping all the supply-chain drama (suffice it to say DEF CON and Entropic, this year’s badge maker, had contract disputes), this is why the dev was removed:

Despite all that disagreement, DEF CON said it was still willing to have Grinberg, who is not an Entropic employee, join a panel to discuss the badge’s concept and development – at least until it came out that Grinberg slipped an Easter egg into the badge firmware during the Entropic/DEF CON beef as a protest.

Pressing the FN button to open a menu, selecting ‘About’ and pressing ‘A’, then pressing ‘Select’ will show the egg. It’s understood that Grinberg didn’t like that Entropic had been scrubbed from the device, the fight over payment, and all mention of the biz working on the badge dropped from the show’s publicity, and so had the egg included to display Entropic’s logo and ask for Bitcoin donations.

The event organizers were not happy that an unexpected feature, particularly the call for donations, was added to the software.

Regardless of who you think was in the right, this was terribly unprofessional and looks like the covert addition of adware to a product even if you personally wouldn’t call it adware. Doing it without the customer’s knowledge puts it firmly on the wrong side of the fence.