Tit for tat is a fantastic model, much like GRIM[1] because it has the advantage of being simple, clear and easily checkable. The EU as an organisation is meant to fight, as one, for its member states’ interests. Easily verified policy makes coordination far, far easier, as does legal and organisational equality. The U.K. expected the EU to throw Ireland under the bus when it came to Brexit and they didn’t because it’s a partnership of nominal equals. For the same reason the EU is extremely strongly motivated to advocate for all its citizens to be treated equally by the US.

[1]Never forgive, never forget