when they want to because you can't do any thing about it
When you get 7 badges at least i think
Chat with our AI personalities
You need to get a Team Rocket-outfit which can be obtained in the underground road in Goldenrod just south of the Radio Tower. After obtaining this you can access the other levels of the Radio Tower. After that it is pretty straightforward.
At the radio tower after you save it from team rocket.
You get it when they take over the radio tower in goldenrod city one of team rocket members gives it to you
after beating the seventh gym leader go to the basement of golderod city. Their is a man by a camera in a team rocket uniform. talk to him and he will give you a team rocket uniform. then talk to the team rocket grunt blocking the stairs of the radio tower. (sorry if I spelt the city wrong. it's in the same place as the radio tower)
During the team rocket radio tower incident in Goldenrod City, you are not allowed to enter the Radio tower because it is blocked by a team rocket member. To receive the Black suit (team rocket costume) go to underground and towards the photographer area (Northeast section). Talk to the Team Rocket member and he will give you the Rocket Suit.