It's not at all illegal, but it is against Google's terms of service, meaning they could potentially deduct clicks or drop you from the program.
The reason for this is that if you ask visitors to click an ad to "donate to the site", they're just going to click any random ad and immediately come right back. They are very unlikely to actually buy anything as a result, so the advertiser is just throwing money away. On the other hand if they click an ad because they see it on their own and it happens to interest them, the click is much more likely to lead to a sale. Advertisers are paying you for space on your site and targeted traffic from it because they want to make sales--they're not charities who want to pay you for the heck of it.
I've run AdSense for 2+ years now and have made a LOT of money from it. From my experience the most effective way to get the most out of it would be to put your ads in a more prominent position, possibly with brighter coloring. Putting a horizontal box across the top of every page would make a DRAMATIC improcement. That way a lot more people will see them and you'll send many more (legitimate, unsolicited) clicks to the advertisers.