Why is a Muslim who knows the Sharia ruling considered sinful if he commits a sin, while one who is ignorant of it is not, and does this contradict the obligation of seeking knowledge?

Knowledge is of two types: an individual obligation (fard ayn) and a communal obligation (fard kifayah).

The individual obligation is that which one must learn to correct their creed, worship, and transactions, such as the rulings of prayer and fasting.

The communal obligation is that which worldly affairs cannot do without, like medicine and arithmetic.

Ignorance may be an excuse in some cases, but negligence in learning what is obligatory incurs punishment for its perpetrator. So, if someone commits a forbidden act out of ignorance, there is no sin or punishment upon them. However, if they were negligent in seeking the truth, they are sinful.

The excuse of ignorance is not absolute. One who was able to acquire knowledge but turned away from it is not excused. If a person had the means to know the truth but fell short, they are not excused. If a person is ignorant of matters commonly known by most people, such as the prohibition of adultery or killing, their ignorance is not accepted unless they are new to Islam or grew up in a remote desert where rulings do not easily reach them.

If the ignorant person were absolutely excused, then ignorance would be better than knowledge, which is not sound Islamically. Therefore, one must seek knowledge and strive in it, and beware of turning away or being negligent, for that leads a person into sin, and they will not be excused by their ignorance.