They may just be concerned about ticking a box that your product has 24/7 support. In that case you could charge a smaller retainer and a large per incident fee. This would align the value they recieve with what they have to pay. It also incentivies them not to bother you unless it is a critical issue.

If they do require a flat fee, I would try to cap the number of incidents. This will force them to place value on support requests and not take advantage of the SLA.

You'll need to be explicit when defining the scope of the SLA. If there is an issue with your product that is impacting production, that seems like a reasonable support request. Alternatively, if they want consulting services or training that may be beyond the scope of the SLA. I've had expierence with customers who open a support request, only to request consulting services.