Standard 24" BMXs come with short seatposts. When you're at the skatepark, this is a good thing; when you're commuting, it's not.
For commuting, you need an extra-long seatpost. Most normal seatposts max out at 400mm in length. To commute on a BMX, we need to go longer.
For comparison, here are some folding-bike seatposts:
Brands known for making longer-than-average seatposts: