Nothing crooked about it. Just perfectly normal political fundraising that all candidates do. You pay a bunch of money, and you get special direct access.

Running a political campaign for president is enormously expensive. Every rally and event is not free: your campaign is covering the entire cost. The money has to come from somewhere.

Rumour is that the whole reason Trump held a rally outdoors – nearly getting killed in the process – was to save money on a venue. A field is cheaper than an enclosed stadium.