Is a "donation" really a "donation " when it is a suggested donation ? :)

I am always amused by this term "suggested donation". Sounds like an oxymoron to me. Either you ask a person to pay a fee or leave it to him to donate whatever he wants.

The only reason that comes to my mind is that a yoga businessman may want to make money from providing the serivce and at the same not want to turn away people who cant afford to pay the stipulated fee. 

Any ideas?