Here's why:

* Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain used in the Gram staining procedure.

* Acid-fast bacteria also have a thick peptidoglycan layer, but they also have a high content of mycolic acid, a waxy substance that makes them resistant to traditional Gram staining.

* The acid-fast stain uses a more aggressive staining method, including heat and a dye that penetrates the waxy layer, to identify these bacteria.

So, while acid-fast bacteria don't stain well with the standard Gram stain, they do have the characteristics of gram-positive bacteria due to their thick peptidoglycan layer.