The product name, Romex, is a trademark. It is owned by Southwire (another trademark). They have used it since 1923. You can see their registration here.
There is a social purpose for trademarks. They were developed to protect consumers. A controlled trademark or guild mark assured the buyer that the stuff was up to some standard (controlled by the mark's owner or guild). If counterfeits came along, they were shut down. The idea is that a shoddy manufacturer or seller cannot fool buyers by using a well respected product's trademark. A secondary purpose is to protect the interest of the respected manufacturer. This is generally called goodwill.
If true Romex® develops a reputation, crappy stuff should not be able to call itself by the same name, fool the buyer into getting wire that does not have the durability or the pull ease or some other characteristic, as the brand name. If the competing product is just as good, it can establish its own brand and emphasize its better price.
Yes, calling products by the leading brand name may be convenient, and yes, I know you think there is no harm, but there is. A trademark owner must protect (called policing) its mark or it loses control over it. Once that happens, it loses its right to own it and protect it. When the brand becomes so common that it is substituted for the product category name, it is said to become generic. When that happens, consumers are hurt because they lose that quality assurance, and the company is hurt because its efforts to distinguish itself are lost.
What is legally required is not to use someone's trademark without permission. Whether or not Southwire decides to sue, the right thing is to follow the rules.
These restrictions apply to competing brands, and also to people and companies who distribute information about the products in question. The restrictions do not apply to individuals in their everyday speech.
At SE, we are disseminators of information. And we hold ourselves out to be experts. We need to avoid confusing the public by mislabeling generic non-metallic cable as Romex® (or any other generic by the category leader's name). And as our reputation and scope expands, we become a more powerful agent of information. If we misuse the brand, it will become generic that much more quickly. We become part of the problem.
@Tester101 has an excellent approach. I respectfully disagree with @Mazura.
P.S. As noted in my earlier comment, some trademarks have been lost because of lack of policing, such as elevator and linoleum. Many other brands are very firm in policing their marks, such as Kleenex. You will note that competing brand of facial tissues call their products facial tissues, not Kleenex. If they did, Kimberley-Clark would sue.