Sorry Phil, you are wrong. e.g. Working in France without a work permit or visa (a la non-EU citizens) will result in a visit by the inspecteur du travail accompanied by the gendarmerie.
I know of at least 1 high tech French company who tried it on with Indian engineers who were over on business visas but actually working on a live project "for a few weeks."
After a snap inspection (Literally, "Show me your papers!") the Indians got thrown out and the company was fined.
I work for an international company that has engineers from US, India and the EU often working in one anothers countries. A few countries, India is an obvious one, insist on an expensive Business or Work (they are different) visa to travel to them, and so other countries tend to reciprocate for Indian passport holders. UK travel to the US and Canada doesn't require a visa for short-term business purposes, I've been doing that for 25 years without problems.
In the rare cases when you need a visa there is no problem if you have the correct one. Your comment that the visiting Indian staff were 'actually working on a live project "for a few weeks."' sounds like the French company was, as you say, trying it on with the wrong visas (which are significantly cheaper), and got caught. Their own fault.
That is the future for Brits and that is perfectly understandable.
It's extremely unlikely to be the future for Brits, and certainly not within Europe. There's no reason for the UK to apply such draconian rules to workers from EU countries (it has already said it won't), and so no reason for a tit-for-tat reaction from the EU.
Just because the UK doesn't do things properly, doesn't mean other EU countries don't.
By "properly" you mean "deliberately awkward and difficult for foreigners" you mean? That's certainly the French way of doing things (especially for foreigners are aren't the "right" colour) but generally not a UK trait, happily.