Spain is like phone companies: a welcome mat for outsiders, a nightmare for insiders. Spanish companies can't have anybody working (i.e. 'dormant' state), they always need an "administrador" who pays a monthly 300€ self-employment tax. This kills companies in early stages unless they can break even quickly (3600€/yr while not having any income, plus maintentance expenses, is terrible) Creating a company is bureaucrat…
I agree that the 300€ are a big turnoff initially, but it is not a self-employment tax. It's social security contributions and it entitles you to full health coverage (and eventually a pension). Granted, you can get the same in the UK for almost nothing until you actually make money. But the 300€ is about the same as in Germany I believe and you get a big rebate in the first few years if you register as self-employed…
Then as long as you don't earn more than 5/6ths of your income from a single client you can remain exempt.
Fall foul of that 5/6ths though and you have to pay full whack (half for the first 6 months) which is a whopping 500 EUR per month.
It is designed to discourage people who really 'should' be employed, working on long term contracts for a single employer.