> I do not think it is as simple as calling it theft in the same way as copying books

Aside from the incentive problem, there is a kind of theft, known as conversion: when you were granted a license under some conditions, and you went beyond them - you kept the car past your rental date, etc. In this case, the documentation is for people to read; AI using it to answer questions is a kind of conversion (no, not fair use). But these license limits are mostly implicit in the assumption that (only) people are reading, or buried in unenforceable site terms of use. So it's a squishy kind of stealing after breaching a squishy kind of contract - too fuzzy to stop incented parties.