1) In many countries, unlike USA, a majority of people don't ever handle a "tax return" or "file taxes" - if your income consists of standard salaries or student stipends or social security/etc, then the employer is required to handle everything and you can generally ignore the issue unless you start doing business or selling real estate or whatever. For a normal person, the cost of starting to file taxes (and investigating how it needs to be done) it's orders of magnitude larger than any reasonable amount of tips - generally, hiring an accountant would be cheaper&safer than doing it yourself and any non-standard income less than the accountant fee is not worth the hassle.
2) Those tips are strictly different from personal gifts - they're payment (voluntary, but still) for stuff you did, so tax wise, they would be considered equivalent to commercial export of software. Taxes on international commerce are tricky (e.g., determining if value added tax applies in this case, etc), most jurisdictions have some simplified paperwork, guides and assistance for small business with the expectation that they're domestic, not international.
3) If you're asking "which specific law prohibits" - it's entirely opposite, it's not like there are specific laws for each taxable thing - the general law mandates that any and all income is declarable and taxable, and then there are specific laws to make exceptions for things such as birthday gifts from relatives or tips for waiters.
Generic endorsement of this comment, coming from someone who has an international business and has occasionally had to decline tips, for these (and other [+]) reasons.
I once had a gentleman attempt to put a rather small sum of money in my hand. Call it $50. He meant it as a nice gesture, because he had been consuming my professional output for years, and it had helped his business. He could not have known, but causing me to have US-source income would have potentially exposed me to several thousand dollars in additional tax liability during that year. (Long story.) There exist ways around it, but at the very least I would have had to loop my accountant in on the incident, so we could discuss whether I have to worry about the $50 "donation."
My accountant's rate is $10. Per minute.
All of this was going through my head when I attempted to gracefully decline.
[+] Totally separate from tax/compliance, I have social reasons why I prefer to not get tipped for things. Social expectations are a weird thing, but regardless of their intrinsic weirdness they appear to be real, and as a result I try to hew to them unless I have a really strong reason to not to. One feature of social classes in the US is that the one I aspire to be in does not accept tips. Tips are something which flow from the relatively well-off to the relatively poorly-off, and even in those relationships where I'm relatively poorly-off, lumping myself in with waiters and massage therapists doesn't sound advantageous when I hope to have a professional image closer to that of a doctor, lawyer, or investment advisor.
For related reasons, I remain very concerned that many OSS developers appear to think that they should have to throw hundreds of hours of professional labor into projects used by for-profit companies so that they can be allowed to participate in Internet busking on a scale which would be sneered at by actual buskers.
2) Those tips are strictly different from personal gifts - they're payment (voluntary, but still) for stuff you did, so tax wise, they would be considered equivalent to commercial export of software. Taxes on international commerce are tricky (e.g., determining if value added tax applies in this case, etc), most jurisdictions have some simplified paperwork, guides and assistance for small business with the expectation that they're domestic, not international.
3) If you're asking "which specific law prohibits" - it's entirely opposite, it's not like there are specific laws for each taxable thing - the general law mandates that any and all income is declarable and taxable, and then there are specific laws to make exceptions for things such as birthday gifts from relatives or tips for waiters.