As I see it there's no force involved, since there will always be a free version.

If someone is willing to shell out for an enhanced version, what does it matter whether they donate, or buy/license the software? Personally, I think it's better for him this way in the sense that everyone is just a customer: noone can attempt to pressure him into any "special interest" features, like they might feel they were entitled to if they were a donator. I'm just playing the devil's advocate with that last statement.

In any case, this is an awesome product and he deserves compensation for it, in whatever way he decides.

Just my $0.02.