snakes casting scare on my troops

[easytarget]

A follow up question occured to me playing last night.

When I have scare or plague cast against me, it looks to me like they've hit every one of my guys with it.

Yet when I as a mage chose to do something (like say, cast slow) it only affects the particular enemy unit I cast it against, not the whole opposing side.

Am I missing something here? Because I'd love to cast these things against everyone, or buff everyone on my side, not just one guy.