I understand what you mean, though they likely don't need DPI. There are simpler methods most websites use to block VPNs: IP address lists, port analysis, detecting multiple connections from single IPs, etc.

And to play devil's advocate for a second, it's not just financial services and streaming services that do this to prevent fraud and unauthorized use/abuse of their services, even some elite private torrent sites block VPNs to prevent unauthorized access, sharing of exclusive releases, and abuse (which results in immediate bans).

AFAIC, it's their site, they can do as they please, and people can choose to not agree to those terms of service and not use the site as they please. Freedom works both ways.