There are far fewer times in polo where you reverse your bike... so the intent is much easier to see (as opposed to deliberately forcing a player off their bike when moving forwards).
As a ref you're calling a t-bone as opposed to a block (or check), there's loads to consider:
Who has possesion?
What was the intent of each player?
Were both players looking up/being observant?
Were the lines of play observed (was an exit point given)?
There are far fewer times in polo where you reverse your bike... so the intent is much easier to see (as opposed to deliberately forcing a player off their bike when moving forwards).
As a ref you're calling a t-bone as opposed to a block (or check), there's loads to consider: