Goalkeepers discriminated against again. A penalty is exactly that. A penalty is:-
(in sports and games) a handicap imposed on a player or team for infringement of rules.
The opposition have been denied an opportunity to score a goal. During that lost opportunity the goalkeeper would very much be able to move and he might very well have been able to deny the opportunity. The opponent might have been going away from goal, been at a tight angle or faced with another three players to get past.
On a penalty, the opposing player, 100% a specialist in scoring goals, is entitled to a free shot unimpeded from 12 yards in the centre of the goal against a keeper already restricted to his line. The entitlement to a free shot with a stationary ball, with no opponents around or in front of him, from the centre of the goal is sufficient handicap to the keeper and his team especially in these days of athletes who can kick the light ball used nowadays at a speed far faster than any keeper can react.
So now a keeper can't move? Just award the goal, you may as well. In fact just outlaw keepers entirely. Inconvenient contrary bastards stopping goals, how dare they.