Another forum I use have a similar set up but the donation is exactly that and an option too, with no restrictions on what you can see or not as the case may be.
HOWEVER, there restriction is you cannot post in the for sale section unless your a regular poster with a lot of posts, i.e +50 posts.
That forum offers fantastic value for money and a donation was provided.
I will donate if I feel the site is worth it given the 'whole' content. There are plenty more sites out there that don't have this so I have the option to go elsewhere.
The fact I cannot access the for sale section is no loss to me, but if the whole site was accessible without the necessity of this silly rule then I would be obliged to donate out of courtesy and not 'forced' into it just to view that particular thread.
Other than that it's a top site
with some top people on here who offer some top advice.