Question

Why were many Northerners upset with the Compromise of 1850?
They disliked the adoption of a strict fugitive slave law.
The Compromise restricted freedom of slaves in the South.
It seriously affected the labor supply of Northern manufacturers.

Answer 1

The main reason why many Northerners were upset with the Compromise of 1850 is because "They disliked the adoption of a strict fugitive slave law," since this meant that any northerner who did not assist with the capture of a fugitive slave could face penalties. 

Answer 2

Many Northerners were upset with the Compromise of 1850 because they disliked the adoption of a strict fugitive slave law.

The compromise was drafted by Whig Senator Henry Clay of Kentucky and negotiated by Clay himself and Democratic Senator Stephen Douglas of Illinois. The controversy that deeply aggravated the crisis arose from the attitude that the northern states had towards the fugitive slaves, since there was an increasing disposition on the part of the northerners to avoid the Fugitive Slave Act of 1793, which obliged those slaves who escaped from the southern states to be returned to their owners. The response to the pressures of the southerners ended in unjust captures of free African Americans who were dragged to the southern states to be subjected to slavery.

The Compromise of 1850 aggravated the situation of fugitive slaves, which was much criticized in the northern society.