The left won but it didn't have an absolute majority, not even close, macron got second place and the far right got third. Ultimately, the president has to appoint a prime-minister (a government) to be approved by the parliament.

How it usually goes in france is that even if the president is not of the same party as the largest party in parliament, he'll still appoint a PM from that party and you have what's called "co-habitation" between the president and government sharing power.

Macron instead of doing that made a deal with the far-right to get their votes in appointing a center-right (not from the far-right party) prime-minister instead. So, ultimately yeah he probably has the right to do that legally but when you see people calling that outrageous they're making a political statement that macron preferred to share power with the far right than with the left