I suspect this is because Theresa May didn't want the UK policy to be so blatantly discriminatory, but still wanted to obsequiously support Donald Trump in the hopes of receiving preferential treatment from America as the UK severs its ties with other EU member states.

Whatever the intelligence says about the risk of certain devices, I can't see how that threat doesn't apply equally to passengers travelling from any country (although plausibly there could be significantly increased risks relating to passengers of certain nationalities).

This policy feels like an obvious target for someone to crowd-fund a judicial review into.