It sounds to me that the HOA decided to submit the cleanup bill, rather than handing down an assessment. Either way, it may not be right, but if you want a respectable and clean development to live in, then pay the bill.
If the $300.00 is a problem for the homeowner, then alert the person in charge of the HOA and set up a payment plan. I know that if it were me, I may not be in agreement with the plan, but I would appreciate living in a clean, eye-pleasing development, especially if I had children.