Is it legal to use tourist visa waiver for academic visits? All countries have visa waiver for certain nationals, but it is only for touristic purpose. People use this scheme for attending scientific meetings (isn't it some kind of business trip?).

Anyway, academic visits, in which the guest is paid, is definitely a business trip. For example, when an academic organize a paid two-week course in the host university, or when visiting for a join project in which he is paid.

For example, a German academic traveling to the United States for two weeks for working on a NSF-funded project in which he is paid.


Is it theoretically illegal to travel without Business Visa for such academic visits?
If yes, how often it happens in practice?
Each country might have different rules, but in the example you're considering, you can benefit from the US Visa Waiver program for business purposes if you would qualify for a Business Visitor Visa (B-1):

Lecturer or speaker: No salary or income from a U.S. based company/entity, other than expenses incidental to the visit. If honorarium will be received, activities can last no longer than nine days at any single institution or organization; payment must be offered by an institution or organization described in INA 212(q); honorarium is for services conducted for the benefit of the institution or entity; and visa applicant will not have accepted such payment or expenses from more than five institutions or organizations over the last six months.
Researcher: Independent research, no salary/income from a U.S. based source, or benefit to U.S. institution.

Researchers who receive US payment should apply for a  Temporary Worker (H-1B) Visa or Exchange Visitor (J) Visa.
So, to answer your first question: if the guest is paid (other than an honorarium, and in addition to have his/her expenses covered), then travelling on the visa waiver program is theoretically illegal. Otherwise, if could be legal. Check with the legal team at the US institution.
About whether it happens in practice, I doubt you'll find plenty of people who are happy to claim they are doing something illegal.