The issue is whether the use of ™ with another's mark is required by law or is it merely customary.
The rule is that if you claim rights to use a mark, then you may use the ™ designation to give notice to the public at large of those rights. Thus, the main purpose of using the ™ designation is to give notice of your rights in the mark to the public, which then consequently boosts the amount of damages in an infringement lawsuit.
However, there is no rule regulating the use of a similar designation when using or mentioning another's mark because the test in that case is to see if the general public would perceive the marks to be too similar to be confused. Such liability may not be exculpated or limited even with the use of the designation such as ™ (for a non-registered mark) or a ® (for a registered mark). However, it might be one of the factors that would be weighed-in when fixing the amount of damages.
Thus, it is not required to use ™ when mentioning or using another's trademark and is merely a customary practice to reduce the amount of damages in an infringement lawsuit.