CE Standard Certification for RepRaps

In the US unless the gear is in a class that is governed by a spurious emission standard, for example a computer used in a home environement, testing and certs aren't required. To my knowledge a 3D printer isn't yet in that class. Over here it's more the application of the completed product rather than the parts that determine what, if any certs or evaluation any particular piece of equipment must go through.

About UL, it's not a law that you must have it. UL is a for profit independent testing company.

This is from UL website
"Do I need to have the UL Mark on my product in the United States? Is there a law stating that my product should have a UL Mark? Does our product require UL testing?
Manufacturers submit products to UL for testing and safety certification on a voluntary basis. There are no laws specifying that a UL Mark must be used. However, in the United States there are many municipalities that have laws, codes or regulations which require a product to be tested by a nationally recognized testing laboratory before it can be sold in their area. UL is the largest and oldest nationally recognized testing laboratory in the United States. UL does not, however, maintain a list of the jurisdictions having such regulations.

If you plan to market your product nationally or internationally, it is advisable to obtain UL Listing. If a limited marketing program is anticipated, check with the municipal office having jurisdiction in the particular areas to learn the local retail ordinances or product installation requirements applicable in that area.

Many companies make it their policy to obtain UL Listing not only to minimize the possibility of local non-acceptance, but also as a matter of corporate policy and commitment to minimize the possibility of risk in the use of their products."

Some local codes and the NEC specifically require parts or completed assemblies to be listed. This is typically in the building trades, entertainment technology and power distribution. Most jurisdictions that require listed devices will accept UL or CSA or CE. Again, in the US it's not the sale of the parts, it's the application. In the EU it's different and some goods are required to be listed/certified prior to sale.

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A2
Maybe there will be a challenge through the courts one day but, in the meantime, plug-in boards such as Raspberry Pi need to be compliant.

Actually they don't!
They don't fit in these product groups CE_marking
(the low voltage group concerns equipments using between 50 V and 1.5 kV DF" target="_blank" rel="nofollow">DIRECTIVE 2006/95/EC)