I think it goes like this...

If you go on an ally's land without Military Access agreed the Senate tells to to get off...

If you are not a Roman faction nobody says anything...

The game effect is apparantly you annoy another faction by leaving your troops on their lands. This is a hidden rating used in deplomacy and effect the likelihood of that faction agreeing to your terms in a deal and can increase the likelihood of them chosing you as their next target...

So if a faction is letting it's troops march all over your lands they are stating they do not care what you think of them... And if they also refuse to ally with you then they are eying you up as their next target...

You can discourage wandering armies by placing your own armies next to them and shadowing them on your lands..

As for a "Get off my Land" diplomacy statment, I think that would be great...