Can I force companies to accept a SEPA bank account? Are there any European regulations I can slap on their faces?
I am not sure if you can force. Any change in regulations to this effect much come from the Company law Board [or equivalent in each countries.]
Right now as much as I understand this is more voluntary for the companies to adopt it and the rationale is its easier for companies and hence beneficial.
SEPA Direct Debit is more recent. The key aspect in this is "Mandate Management". Before SEPA every country had slightly different norms. Some were fundamentally different, for example in Belgium Mandate was to be captured and stored by your Bank [payee Bank/Debtor Bank]. In SEPA it is the responsibility of T-Mobile [Creditor].
Although SEPA has guidelines, they are more elementary. For example "Mandate Signature" needs to be on paper. Can an electronic copy be created? Belgium has some guidelines. Other countries have different. This falls in preview of each country defining what a "Signature" is, what is valid electronic signature etc.
Further SEPA DD rules allow you to reverse a debit with 8 weeks [you can inform your Bank in Netherlands]. There is also a period of 13 months where you can say this debit was unauthorized. Now this is where is risk comes in as it is borne by the Creditor [T-Mobile].
Depending on convoluted rules in different countries, you may easily prove you didn't sign the mandate because as per Netherlands law, Signature / authorization means xyz.
This is more likely the reason the Service providers are not willing to accept account in other Euro Zone.