Just coming in here to say this is not true and has never been true - none of the Regis can be encountered in the Market section. I'm guessing you made a mistake.
Yeah. I was on WiFi when I caught him, with no data, and when I was going to post this, my WiFi disconnected. I have no data until the 13th
Same here - RP Institute has quite a few Regis. Encountered four Registeels, another Regice, and Regigigas.