Hmm. That does seem like an odd fit in the Libertarian party. Why would GJ like him?
My guess? Gary Johnson is no fool. He knows the Libertarian voter base is small (maybe 300,000 registered Libertarians). He knows with just Libertarian voters, he'll go nowhere.
Bill Weld is the former governor of Massachusetts (Republican). He's more "mainstream" than other Libertarians, which means Johnson can attract considerably more voters. Johnson is likely banking that more "mainstream" Republicans will be attracted to his ticket.
Weld has stated he's "converted" to being a Libertarian, and he no longer holds "all of his past views", but many Libertarians, and myself, aren't quite buying it.
Years past, the Libertarian Party was similar to the Green Party in that they were "nuisances" to the Dems and GOP. This year, they are much more than that. Dr. Jill Stein is even gaining momentum.