I was wondering this too, and the only thing I could think of is maybe it's one of those things where they don't have options left on Mayza. I don't really know how that stuff works, but as soon as Pearson was sent down I was didn't understand this move.
"An option (optional assignment)
allows a club to move a player on its 40-man roster to and from the minor leagues without exposing him to other teams. Once a player is added to a team’s 40-man roster, his team has three options, or three different seasons in which the club may to send him to the minor leagues without having to clear waivers."
A general rule without having to look things up, is if the guy is in this 4th partial year in the majors he will not be able to be optioned without going through waivers. For many its the second or 3rd year, because they didn't make the team the first year or two after getting on the 40 man.
The Jays have an experienced pitching staff so there were only 3 guys that could be demoted without being placed on waivers. (Mayza, Cabrera, Francis)