Well, an 'anemone island' is a freestanding pile of rocks. BTAs tend to move around rock, but won't generally cross sand. So by making a freestanding 'island' of rock, many folks have been able to keep them confined to a particular area.
Now, if a BTA is *really* stressed, by too much flow, bad water conditions, etc- they have been known to completely 'let go' and float off somewhere else. But that's the exception rather than the rule.
Also, by making an 'island', you can generally pick up the rock an anemone is on without disturbing the rest of your rock work- especially useful if you've had one split and want to sell / trade-in the new anemone.
A friend of mine here in Texas has made a pocketful of spending money for his kid in college by doing that.