I suspect many of us (if not most) are old enough to remember the flight deck entry rules prior to 9/11. For those who don't remember, it was pretty common - especially on longer flights - for passengers to be allowed to visit the flight deck briefly, even allowed to talk to the pilots (sitting in a pilot's seat is a whole different matter). I recall an Air Canada flight from Montreal to Vancouver back around 1990 - I'd gotten bumped up to First Class (they wanted my coach seat so a family could sit together). After a real nice First Class meal, I noticed a young child being escorted into the flight deck by one of the flight attendants. The aircraft was an A320 - still pretty new and novel on the time - and I wanted a chance to check out the flight deck so I asked a flight attendant and she promptly escorted me to the Flight Deck where I introduced myself as a Boeing Propulsion engineer and spent several minutes discussing with the crew their A320 likes and dislikes. Before 9/11, this sort of thing during a long cruise was common.
Post 9/11 everything changed - however I can see how a charter flight for a MLB team might be treated differently than a standard passenger flight.
However letting a 'guest' sit in one of the pilots' seats is way different...