A whole load of Mac shareware isn't crippled. A shareware fee could just be placed on it, simple as that.
But I for one, as a matter of principle, would never download a plugin that costs money. You're already paying 30 dollars for the engine to run plugins, don't add more and more money to that.
Also, if one person starts charging money, more people will realize that they can charge and then there'll be a whole line of plugins by people who want to look professional by charging, when they really shouldn't charge for such a crappy plugin (hypothetical, of course).
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-Aubrey