A note of free play live games
Why a few online casinos let you play free, but most don't...

The games offered by the above casinos are all 'one-to-many' live games.  That is, each dealer/table can cater to a limitless number of players. There is therefore no marginal cost of dealing to additional players - whether playing for real money or for free  - so free play is offered.

Casinos that offer games that are not 'one-to-many' will not offer free play.  Classic multiplayer blackjack is an example of such a game.  Many players require many dealers and tables and the casinos don't want seats taken up by players just practicing for free.

For 'One to many' live games a single table/dealer caters to a limitless number of players...

Multi-player live game.  More participating players means more tables and dealers are required...