As XP was withdrawn as a retail package some time ago, perhaps some surplus stock is being given away? But that doesn't accord with the "copies of a corporate master disk" you mentioned.
I must say I am struggling to see how what you describe would otherwise be legit!
I believe that there are 3 kinds of license:
1) Full retail, in which case the disk should not be copied by a 3rd party.
2) OEM, in which case the software is only supposed to be distributed pre-installed on a new PC (although this is sufficiently gray as to be open to "interpretation").
3) a volume license agreement, such as a company or organization would use to license all the software on its PCs and servers.
Only (2) above would perhaps be applicable - if the vendor has acquired multiple licenses from MS for pre-installation, but has elected to give them away at no cost. It is still quite dubious as to the legality of using an OEM license in this way.
Can you provide any further information?
SD