"penser" is being used here in the affirmative présent indicatif and as such is not expressing any doubt.
If the verb had been negated, thereby expressing doubt; then the subjunctive would have been required because of the "doubt" aspect.
Therefore "sont" is correct.
Hope this helps.
I long ago learned with respect to the French subjunctive to respect the rule "usage fait loi" and listen to what I'm saying rather than trying to implement rules. Famously, espérer doesn't usually take the subjunctive. I speak French daily and find myself correcting what I've said after it hits the air. My French interlocutors tell me I always get in right either the first time or when i notice it doesn't sound right.
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