The answer might be already hidden in the answer to this question: In the US, there seems to be no standardized norm for sleeping bags, i.e. every manufacturer can write onto the label whatever they want – which can be more or less realistic... Basically this means that they will possibly write the most impressive number onto the label they think they can get away with.
If you want to sell your bags also in Europe, they have to be certified according to EN 13537 which is based on a standardized test. One might argue about how realistic that test is but it should* provide at least equal conditions for the temperature ratings across different products and manufacturers.
However, this certification is surely not cheap and therefore manufacturers (especially smaller ones like Feathered Friends) might abstain from it due to cost reasons, especially if they don't target the European markets anyways. Larger manufacturers (such as North Face) might use the greater freedom of the US arrangement to produce a bag for the non-European market that can be slightly cheaper without the EN certification.
*Obviously there are significant deviations and measurement precision problems among different testing labs which seems to spoil this comparability, as can be read from this report some major manufacturers commissioned.