Obviously so, as in the case of eating meat on Fridays. But when they do so, it only pertains to Catholics.
Again, perhaps you misread the article. This is a discipline. The sin aspect enters when one applies the conditions for a mortal sin, as defined above.
This is all based on scripture. Consider Mark 12:30. From the KJV:
And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment.
The one thing I know for a fact is that my non-catholic brethren in the forum LOVE our Lord, Jesus Christ. How do you show it in daily life?
One way we Catholic show our love for Jesus is by making a personal sacrifice. Have you ever done that? Have you ever offered up a personal pain or discomfort out of love for Jesus? He gave His life for you, out of love!
The Catholic Church encourages its members to reciprocate that love by making the personal sacrifice of giving up meat on Fridays, the day on which our Lord was nailed to a cross, out of love for us. Of course, it is not much of a sacrifice if one substitutes a lobster dinner for a t-bone steak. The notion is one of personal discipline.
So what is actually accomplished by following this discipline? Personally speaking, on Fridays, I CHOOSE to follow this discipline out of total LOVE for Jesus. There have been times when I was hungry and the only food being offered at an event I was attending, was meat. My focus remained fixed on Christ. He suffered and died for me and I can certainly forego a hamburger, offering up any discomfort, out of love. Of course, there are many Catholics who do not understand the concept of self sacrifice. For them, the Church has provided the option of substituting some other form of personal sacrifice.
Regardless of whether or not you understand or agree with this concept, it is driven by love. Christ is ever present in my life but on Friday, the relationship tightens and becomes very close knit: I do this out of love for my Savior!
BTW, this is a Catholic Caucus thread. Out of love for YOU, I am offering this detailed and very personal explanation.