Just my own .02 cents worth thrown in for perspective.
Paypal has a notorious habit of suspending services when anomalies present themselves. For example, all electronic transactions have a multiple back & forth "conversation" electronically as part of the transaction process. For credit cards, this is called a "pre-authorization", which explains why a transaction of $25 sometimes shows up temporarily on your online statement for $150. The retailer has no input on the amount of the "Pre-Auth", that is determined electronically by the banking institution.
For PayPal, their electronic communication hasn't always been up to the same standards as those of banks that issue credit cards, and therefore, anomalies occur. As with any banking institution, when an anomaly occurs, the first safeguard is to suspend services pending confirmation against fraudelant activity.
For Paypal, this "suspension of services" doesn't limit itself to the consumer only, but also the retailer, hence why not every retailer offers PayPal as an option.
For a retailer, the suspension of a credit card only delays a transaction on the consumer end, but to have transactions as a whole using a particular payment method suspending on both the consumer and the sellers end, can wreak havoc on the seller, especially when dealing worlwide with things like currency exchange rates & such, and then compile that with perhaps several times a day or week, for a small business.
I can see why many businesses don't deal with PayPal, especially small businesses such as Warmoth. Larger businesses can negotiate terms with an entity as large as PayPal, but a small business has little or no negotiating leverage.
Let's also put things into perspective, Warmoth is indeed a small business, under 50 employees, all stuffed into an 8000 sq ft pole building with the world's largest inventory of replacement bodies, necks, the largest Floyd Rose, Seymour Duncan dealer, file cabinets, machinery, wood storage, office staff. It's a testament of efficiency for the use of the space that they are using, but it's still a small business.