Sites geared around punter play will be site paint only.
Walkons (true walkons rather than just alllowing in own gunners to play among the punters) may be more open to BYO paint.
The same with events, however due to the extra cost of running an event with more organisation and bringing in more extras then paint sales by the site put the profit into the site and the event, and lower the threshold they need on the entry price to afford ot run the game
With 'event paint only' from traders who attend, it means the companies who put in the mileage and attend the events with tech supprt, parts and gear get a revenue stream rather than just costs and hoping for the odd sale of an item.
Put your paint in the hands of a courier and you risk extra breaks. Get breaks in the post and you are cleaning the rest on a towel, find broken paint when you buy on site then you go back to the counter
Unless theres a special paint you need, the main reason people try to BYO is to save money. If you're ordering it in then you are also paying postage (even if it says free delivery) If you can bring paint cheaper then you can buy on site then theres a reason - Probably a combination of cheap basic online paint and the site needing a profit margin. If the profit margin is high then you're playing punter games