As a European, I'll admit that I am uncomfortable with a business model where - in essence - the customer is expected to pay the wages of staff as well as pay for the service provided.
My sense is that if staff were paid a decent basic wage, then the way tips are viewed would be different. This would be because customers would not have the responsibility of having to determine the degree to which service would - or should - be rewarded (which matters when wages are so risibly low), and staff would have the security and stability of a guaranteed wage, and not have to wonder whether or not their efforts will be rewarded irrespective of how good the service they supply actually is.
Thus, there wouldn't be this massive power imbalance, where the employer incurs hardly any costs, the customer (not the employer) is on the receiving end of employee resentment, and whether or not the employee gets paid at all is entirely dependent on the discretion, or goodwill, or just the mood, of the customer.
To be quite candid, intellectually, (and hypothetically), I would be a lot more comfortable in a system where signs are openly and prominently displayed, signs which could state clearly, if with an uncomfortable candour, as you entered the premises, something along the lines of: "We cannot be bothered to pay our staff, as we'd prefer to use every opportunity to squeeze profit out of them instead, and that means it is up to you to give them something by way of remunerative reward if you feel like it, but - and, meanwhile, get this - they are free to make your life miserable with poor service if you don't, or, if you choose not to. Meanwhile, this has nothing to do with me, any of it; I merely supply the space where this can happen."
In Europe, the staff are paid a salary, - below which it cannot go by law - and can be expected to be able to afford a basic - if not massively decent - living on that.
Thus, tips are discretionary, not mandatory, and are usually tendered if service has been very good. That means, that if service is indifferent or poor, you don't tip.
Re credit cards, for that matter, I have heard too many horror stories of how the 'discretionary service charge' may or may not get paid to staff. However, if there is an added service charge on the actual bill, - as there usually is, the kind that is not discretionary - I will not tip unless the service has been exceptional, as I have already been charged for it, and, even then, I will do so in cash - that way the server gets it - rather then on a credit card.
No single website or book can explain it fully. Also, OP, most tour guides expect a tip. Bus tours or biking tours, that sort of thing.
