From what I understand, there's no law because there's no need for a law as strikebreaking isn't common, and these sorts of issues are covered by industry-wide trade union agreements and protected by the broad right to strike.

While in the US employing strikebreakers has been increasingly common since the 1970s, and there is only a much more limited right to strike. (Eg, sympathy strikes, like the Danish McDonald's one mentioned at https://news.ycombinator.com/item?id=38036371 , are illegal in the US due to the anti-worker Taft-Hartley Act.)

The US court interpretation encourages strike breaking. For example, when combined with the decertification provision of Taft-Hartley Act, you can hire strikebreakers then have the new population of workers decertify the union. (This is one of the examples at https://en.wikipedia.org/wiki/NLRB_v._Mackay_Radio_%26_Teleg.... .)

So even if strikebreaking isn't illegal in either country, the legal framework which protects strikebreaking is stronger in the US, and the legally allowed consequences of strikebreaking are weaker.

These is part of the legal framework which a US employer should learn and understand when expanding to Europe.

To give what I think is a reasonable analogy, Sweden does not have a minimum wage law while the US does. Instead, minimum wages are determined by union agreements on an industry sector basis.

A US employer who enters Sweden and offers a position for only $5/hour might consider that reasonable, as there is no law against it. However, they would (as the Denmark/McDonald's case shows) be subject to industry action that is prohibited in the US.