Checking it down

[mr.spam]

Home game, S&G, first 3 pay, one shortstack on button, 3 players left. Shortie moves all-in, sb thinks for a while and makes the call. BB thinks for a while too and says "Whell, I have a decent hand which can easily knock shortie out too. How about I call and we check it down?" SB agrees but button protests that they can't talk about this and that they can't agree to check it down.
What's the ruling here? How improper behaviour is this? I've heard this before but don't know the ruling about this...