Because you really want to empower your opponent's shadowstrike?

Oh, I see - you've got a cycle of ETBT-unless-a-cretaure-is-tapped lands. Slightly a shame that you have to make this decision before your draw, then.

Maybe this would be cleaner with a very low-power : ability instead? If you want to keep the puzzle-box aspect, may I propose "exhcnage power and toughness" which is pleasingly pointless on a 2/2, but easy enough to find ways to make not-pointless. Oh, except then you have a TAPPED creature with exchange P/T, ok, that is also too too pointless.

There will be a protection spell for all tapped creatures too, to counter shadowstrike. But UG will be tap heavy, flavor is this area is a pollen filled region that causes eternal sleep, those here are protected/put to sleep by the Faerie spirits there. So expect mass sleep also.