Sprinting does not cancel ironsights, rather ironsights cancels sprinting. This is because ironsights is always a hold interaction, whereas sprinting is either toggle or hold. Sort of a universal usability rule. If you find it a huge issue, I'd suggest customizing your weapons so that using ironsights isn't really necessary.
If you tap the button and don't hold it, you'll sprint but not activate SMART.
How do you feel about toggle run and hold ironsights?