My thought is that unsealing Velcro must tear the threads because the material can wear out. Dear Ed, Rabbi Yehoshua Neuwirth in his classical book Shemirath Shabbath Kehilchata writes that it is permissible to use Velcro on Shabbat. Velcro is essentially the hooking of strands on the one piece through loops on the other. Even if some strands do in fact tear, they are an insignificant few, and besides, they are not designed to tear.
Velcro does eventually wear out, but only after a great deal of use.
An analogy would be walking across a lawn on Shabbat. Even though an occasional piece of grass may be uprooted when you walk over it you certainly didn't intend to uproot it. Since it is not certain that a strand will tear, it is not forbidden to use Velcro on Shabbat. In the first one, a coin is thrown into a fountain or the like, but it will eventually be retrieved by someone and given to charity. In this case it would be permitted for a Jew to throw the coin and make a wish, provided of course that the wish is directed to G-d, asking Him to fulfill the wish.