multicast: Linux *must not* bind socket to a specific # but Windows *must* - linux

Question

I have a mdns service discovery that uses the following code for initialization
void mdnssd_init(struct in_addr host, bool compliant) {
int sock;
int res;
struct ip_mreq mreq;
struct sockaddr_in addr;
sock = socket(AF_INET, SOCK_DGRAM, 0);
char param = 32;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, (void*) &param, sizeof(param));
int enable = 1
setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (void*) &enable, sizeof(enable));
param = 1;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, (void*) &param, sizeof(param));
#ifndef _WIN32
if (compliant) {
enable = 1;
setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, (void*)&enable, sizeof(enable));
}
#endif
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
/*
* Sending *from * 5353 indicates that we do compliant mDNS query. If we chose
* random ports, the ttl will be much shorter.
*/
if (compliant) addr.sin_port = htons(MDNS_PORT);
// Windows must bind this socket to a specific address, others must not (it's *must*)
#ifdef _WIN32
addr.sin_addr.s_addr = host.s_addr;
#else
addr.sin_addr.s_addr = INADDR_ANY;
#endif
socklen_t addrlen = sizeof(addr);
res = bind(sock, (struct sockaddr *) &addr, addrlen);
if (res < 0) return;
// set outgoing interface for multicast (it's optional, INADDR_ANY could be used)
setsockopt (sock, IPPROTO_IP, IP_MULTICAST_IF, (void*) &host.s_addr, sizeof(host.s_addr));
// set multicast groups we are interested by to receive such packets
memset(&mreq, 0, sizeof(mreq));
mreq.imr_multiaddr.s_addr = inet_addr(MDNS_MULTICAST_ADDRESS);
mreq.imr_interface.s_addr = host.s_addr; // optional, INADDR_ANY can be used
setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void*) &mreq, sizeof(mreq));
}
It works on all platforms (Linux, Windows, Solaris, FreeBSD, MacOS) but I really don't understand why I need that binding difference between Linux and Windows.
On Linux, if I bind the socket to the address I want to use to send/receive, multicast traffic is not received. Note that queries that require unicast response are properly answered. I understand that some settings here are optional and INADDR_ANY can be used, letting the OS select what interface to send request and receive response (and it works).
On the contrary, on Windows, if the socket is not bound to a specific address, but set to INADDR_ANY, then no multicast traffic is received. Same, queries requiring unicast responses are received.
So it's very puzzling to me that not both options work. I should be able to bind the socket to the address that will be used for sending/receiving - or not. It should work either way, no?