The IOC has registered many of its Olympic symbols as trademarks or designs. In addition, there are often local laws to prevent any kind of Ambush marketing. Only the official sponsors are allowed to use the Olympic symbols (like the words OLYMPIC or the 5 rings). So basically, all signs referring to the Olympics are forbidden to use, regardless of whether they are registered. The same rules apply to posts on social media. Private sponsors may congratulate athletes only if it aligns with Rule 40.