Earlier quoted context omitted.
Trump changes plenty, in the past it was assumed that the USA would honor visa once they were issued, now it turns out not to be the case. Essentially this is breach of contract.
It is not a breach of contract. A visa is not a guarantee of entry even though many people assume it is. [Edit: Please don't interpret my comment as suggesting I think this is a good idea. If you check my posting history, you'll discover I'm an outspoken proponent of open borders.]
Even so, if my country started to structurally refuse entry to foreigners who had - at their expense and with considerable documentation requirements - obtained a visa in the legal way I'd be quite upset and in fact highly surprised.
See, even though a visa is not a guarantee the general rule is that they are not withdrawn capriciously and if you do that you risk making people from other countries wonder if it is worth applying for a visa in the first place.