Why wouldnt celebrities be able to fly on passenger planes?
If there was some security risk to consider, it would still be cheaper by a factor of 100 or more to just buy a business class ticket and buy some tickets for the security guards too. I am sure, there is also options to organize not going through the public check-in of the airport and to be able to board the plane discreetly before the normal boarding.
Also it is very easy to defend someone on a public plane. the aisles don’t allow for more than one attacker at once, and i am sure there is enough tall muscular men for hire to block all aisles.