Bright Look and "Come into Play powers"

[Kenshin's Garde]

So I have a question on this ruling: http://pokegym.net/forums/showthread.php?t=139784

Why MUST you switch the Defending Pokemon when you declare Bright Look? The "may" clause in the poke-power seems to imply that, not only is the use and activation of the Poke-power voluntary, but the actual use of the effect is temporary as well.

 

[PokePop]

The "may" clause only means that you may or may not use the Power.
Once you decide to use the Power, then you use it. You are already past the "may" at that point. There is only one "may" on the card.