Cheryl is a consultant in the energy industry, and she has clients in the US, for whom she does consulting work. She is a resident of the UK. By law, UK residents (and those of other countries; there's a list somewhere) may come to the USA for visits for work or vacation of less than 90 days and not more than 180 days/year without a visa. (This is known as the visa waiver program, and it's the same reason you can I can go to the UK without a visa for similar visits either for work or leisure.) She is not doing anything illegal.

Similarly, if my company needed to send me to our office in Eersel, Netherlands, for a few days or weeks to work on a computer problem, that would be legal under the same program. (Me staying more than 90 days or more than 180 days in a year would be a problem and would require a separate visa application. And the requirements to Australia are different because they don't participate in the visa waiver program. But I digress.)

The maddening part is that the system is set up so that even if you have a visa, the inspecting officer at the point of entry can, without recourse and without appeal, arbitrarily decide that s/he doesn't like your looks and can deny you entry. You may not hear about it happening that often, but that is how it can and does work.