I guess Rapid7 wants to do closed source versions in the future. That makes it necessary to buy the copyrights, or at least most of them and re-developing the stuff they could not buy.
Of course that puts them in competition with whatever open source version other people maintain. A previous example of that would be Interbase by Borland. Borland released the version 6.0 under an open source license, but reconsidered soon after and the next version was closed source again.
That one open source release was picked up by a group of developers and became the Firebird database (http://www.firebirdsql.org/). Interbase is still available commercially from Embarcadero, but I guess they lose some sales to its open source version.