Processing a micropayment takes more resources than serving a simple Web page, even if the payment fails. You would have to put in similar filters, or you would get DoSed using the micropayment service.
What would be the incentive to send failing payment requests?
The sender does not have a direct communication channel with the receiver. They send the payment to a hop they are connected to (they have a channel with) and it gets routed to the receiver. The first hop would already drop an invalid payment. If they spam them with more invalid payments, all that would happen is that their connection to the Lightning Network would get lost as their channel partners would disconnect from them. The receiver would not receive a single network packet in the whole process.