WRT eating meat on Friday, if Scripture doesn't say it shouldn't be done, then why did the Church teach for so long that it was a sin to do it?
The Church never *asked* people to do it as penance. It was a dictate and Catholics were told it was a sin if they did.
There was no choice. It's not as innocuous as you're making it sound.
Jesus committed the keys of the Kingdom, to bind and loose in heaven and on earth to His Church. Consequently, the Church can bind it's members to do certain things under pain of sin. The Church has set apart certain days of the year as days of fasting and abstinence and bound most Catholics to observe them. (Ash Wednesday, Good Friday are currently the only two required days of fasting, while all Fridays of the year are days of abstinence (not eating meat). Thus Friday abstinence from meat is STILL required of Catholics, though at present the Church allows an individual to replace abstaining from meat with another penance if a person so chooses.
I said that the normal way the Church has asked Catholics to do penance is to abstain from meat. This refers to the present time. The Church today STILL asks its member to not eat meat on Fridays, though it allows them to substitute this practice with some other penance. The ability of a person to substitute the penance with something else under present law is why the use of the word "asked" in my sentence was preferable.
You are referring to the older, stricter law in force before the promulgation of the 1983 Code of Canon Law. The older law REQUIRED all relevant Catholics to not eat meat at all, no exceptions or substituting the practice with some other penance. As for the doctrinal justification for this practice, Jesus granted his Church the keys of the Kingdom and the power to bind and loose on heaven and on earth. The Church may use this power to bind its members to do certain things such as fast, pray, or attend Church at certain times.