Sorry for the bad 411, folks. According to
MLB.com, Erick Threets would have had to stick with a major league team if he had been claimed off waivers. So there goes the premise for my previous post. Here's my best understanding of the situation: If a team had claimed him off waivers, he would still technically be under the same obligations as with the Giants, i.e., under their control but out of options. So the claiming team would be in the same bind.
But when he cleared waivers, he was technically a free agent before he re-upped with the Giants, so I don't think anyone will have a 25-man roster obligation if he changes organizations from now on. Please let me know if I'm wrong.
Not only that, but Grizzlies media guy Paul Kennedy informs me that Eliezer Alfonzo is playing in Augusta to serve the final portion of his suspension, not as a demotion. Apparently suspended players get a grace period to rehab before their suspension ends.
It seems that Threets did not have the right to refuse assignment because this was his first time clearing waivers. Every subsequent time he will have the rights to be a free agent. See http://baseballanalysts.com/archives/2006/08/death_taxes_and_1.php, 1st sentence of the last part on Prior Outrights.