Brushing the cable with your foot will eventually cause the excess to break off right at the clamp. It'll be OK until you have to adjust it for any reason, then you won't be able to get the cable back into the clamp again and will have to get a new one. I wouldn't worry about the cable slipping out of the clamp. If brakes did that, they would gradually loosen all the time.
How to prevent the problem in the future: Whenever you have a new cable installed, make sure that the mechanic cuts the new cable at least 3 inches past the clamp. Bend the cable down parallel to the brake arm and secure it with a small zip tie.