Upvote:1
As you correctly point out, the Visa Waiver Program governmental webpage states:
Business
[...]
- attend a scientific, educational, professional, or business convention or conference
- attend short-term training (you may not be paid by any source in the United States with the exception of expenses incidental to your stay)
You should therefore be allowed to attend a conference in the USA on the Visa Waiver Program. You should also be allowed to complete a training period with a company in the US, as long as you receive no money from that company aside from reimburs*m*nts for living expenses.
More importantly however, a distinction is to be made on whether the training is considered an internship or not. The idea behind this being that internships can lead to a potential job offer, hence carrying out an internship in the US would effectively be considered job-seeking. This is specified in the Business Visa Travel to the United States leaflet which can be found on the aforementioned governmental website:
Training
Participating in a training program that is not designed primarily to provide employment. Will receive no payment or income from a U.S. based company/entity, other than an expense allowance or expense reimburs*m*nt related to travelerβs stay.