The difference I called out is that summons, being magically summoned, seem to have some knowledge of who your enemies are. Otherwise, they would attack your allies, right?
Animals, normal everyday animals like would be used with Handle Animal, don't have a necessary knowledge on who is your friend and who is your foe. If "Attack" also doesn't imply "Don't attack unless I point", then you risk your animal charging at any new person you meet.
Attack party members? Probably not if the AC has spent enough time with them. But Attack *needs* to also infer that they won't attack without your command on a specific target. Can you imagine the druid who can't enter towns or use diplomacy on anyone because his lion keeps charging?
"We were planning on attacking them-"
"SIMBA NOT NOW."
Just an example.
Again, *I'M NOT HARD SET ON THIS*. Not sure if you guys saw my above responses, but I ruled for my own game that the druid could point at a specific area that he could smell the enemy in, and the spider with tremorsense could attack.
Would I have let him do that with a creature that had no way to sense invisible creatures though? Probably not. :)