English Heritage was set up by the government to protect historic sites, and then spun out as an independent charity to continue that role.

Protecting sites includes limiting the numbers who can visit, hence enclosing them. That allows visitor numbers to be capped and managed (which reduces damage from over tourism) and also prevents illicit access and vandalisn.

In the case of seahenge it was literally rotting away - the decision was made to excavate and preserve what was left. That was in response to press campaigns to do something to save seahenge; it was a controversial at the time and remains so now. They did this while still part of the UK government in 1999.

Stonehenge was gifted to the nation in 1915 and had been on private land up to that ppint. A lot of expensive work has been done to preserve the site including demolishing other structures to preserve the skyline, and even recently burying a section of road.

Visitor charges and subscriptions pay for English Heritage to continue their work and preserve our history. They're not "robbing bastards", they're a non profit with an expensive role.