From my current limited research, I have come under the impression that some companies do not like it when their logo is modified. For example, in the Twitter Trademark Guidelines (which can be retrieved here) state in rule 1:
You may not alter the Twitter trademarks in any manner, including, but not limited to, changing the proportion, color or shape of the Twitter trademarks, or adding or removing any elements from the Twitter trademarks.
Which brings me to my question: is it legal for sites like Flaticon to distribute social media icon packs that seem to break this rule? Especially since this website has a "no attribution required" subscription perk. Like do they make money off this? If so how are big media companies not all over it?