Maybe that’s your preference, but it certainly isn’t against the law to offer advertising.
But SEO is quite hidden and hard to define. Ads are there, see a banner. SEO? No obvious 100% criteria for it.
Can one tell that a text is produced by SEO? Usually no (especially, when it's good quality).
Now, the difference is:
- In ads case, a government comes and asks for a tax. Because there is a clear case for a transaction in an economy sense (ad for money).
- In SEO case, no transaction proof is there. Can't come and ask to pay tax, or regulate anything. It could be a happy customer who wrote a piece of text. Or clever SEO author. Or both mixed. Can't tell! In this sense SEO brings less or no tax.