You can attempt to attack / move to / cast many spells on arbitrary pixels on the map. The bots are shown casting spells on targets that aren't visible in the demo. The amount of available targets probably blows up the count.
For a hero like invoker who can cast Sunstrike anywhere on the map at any time, would you try to come up with domain heuristics (only consider locations in the map near enemy heroes), or deal with an explosion of possible actions (and this applies to a ton of different hero mechanics that are not in scope here)?
In the development for the Magic the Gathering AI (Duels), one of the restrictions is "don't cast harmful spells on your targets" even though for some edge cases this is actually the optimal thing to do. They traded a smaller search space at the expense of optimality.
Sniper and Viper have non-targeted abilities that they were using to zone the enemy in a teamfight.