Cracked Minecraft servers are Minecraft servers like any other, with one exception. Cracked servers are running in offline mode, thereby skipping authentication on official Minecraft servers and allowing anyone to join, whenever they bought Minecraft or not.
Since you don't need Minecraft account to join cracked servers, those are sometimes kinda similar to wild-west as anyone can create as much accounts are they want. Some servers would limit registrations per IP and so on but on most servers the issue persists.
Furthermore mostly because of the reasons listed above, most Minecraft server operators choose not to deal with all this and would simply turn online-mode on on their server, which would prevent cracked players from joining.