Reason for being non-free: personal use only. no sale/distribution in conjunction with a commercial product.
gxemul can do what spim does, does it a lot better, and does a lot more. Should ask the spim maintainer if spim is needed anymore. UCB SPIM (http://inst.eecs.berkeley.edu/~spim/) was forked from SPIM when it still was GPL (circa 1999?).