You should charge them a small disposal fee. 20 cents a gallon.
If they don't know you use it for fuel then what do they believe you are hauling it away for free for? ****s and giggle? Women's Grease Wrestling?
If you charge a very small amount then you've implied your benefit, and you appear to be a legit disposal company. Someone looking to provide a free service to me at their own expense always raises a flag. It says that there is an underlying benefit that I must not know about if this "company" is willing to haul my "trash" off-site for free.