Complying with the conditions of your contract is absolutely legal. That's actually what is expected of you. If you go beyond what you were hired for that certainly makes you a good employee, but that enthusiasm isn't a condition of your employment.
After all, complying with the contract conditions is exactly what the company does. They even go to great length to make sure that they do nothing more than what's required of them in the contract, as can be seen with the SO bypass pay court case, direct entry captains, etc.
The distinction, and when it may be construed as industrial action, is when a union encourages the whole employee group to work to rule to pressure the employer. That action can be seen as industrial action. But there's a distinction between a union asking its members to go into contract compliance and an individual doing it on its own. Nothing is illegal in complying to your contract and not going above it should you wish to do so on your own.