It’s good to question and get into the detail on investment decisions!
Presumably yes a VC would join at a less risky stage, however, the share price will be higher. I invested in the first round when there was just a business plan, taking on lots more risk than current investors but with the chance of a larger reward. There has been lots of dilution in each of the 4 rounds, but each time my investment has gone up and this is what will continue to happen if the business is a success, it’s now less risky than it has ever been 
Freetrade’s a customer first, community focused and fully crowdfunded business. It makes a lot of sense for it to continue to allow it’s customers to participate in future rounds, regardless of what the terms say - and they even want to do this in future via their app, like Monzo. That being said it’s sensible for it to keep this as optional as in some rounds it might not make financial sense due to high admin costs as per Adam’s (CEO) response.