Flying to CDG via MAD is backtracking. And there are already tons of 1-stop options from BOS to CDG. Besides, if you talk cheap than TP is often much cheaper than IB (e.g. BOS-LIS-CDG).
TP doesn't fly to CDG. They fly to ORY. On top of that the O&D market between LIS and PAR is huge (1st or 2nd). I'm not sure that TP is connecting many people from the U.S. to Paris for those 2 reasons.
But like you said, I think the reason CDG and FCO are not in that list is because SkyTeam has that well covered from BOS plus LH is essentially Italy's second airline
The thing about TP is their cheap connecting tickets are typically to markets where they need to fill seats due to poor loads. Paris is not usually one of those. I guess when you think about it that is how most hub-and-spoke carriers operate.
Here was my logic behind this - BOS-PAR is probably 15-20 times larger than BOS-CMN or BOS-AGP so even with nonstops on LCC's there has to be some trickle down effect to any European hub. A person that doesn't live on this forum has no clue that flying BOS-MAD-CDG/ORY adds over 500 miles of extra travel. Many people asked me, "Did you connect in California?", for my recent trip to HK and Thailand.
Airlines flown: A3, AA, AC, AF, AM, BA, B6, CA, CO, CX, DL, EA, EL, IB, LH, MI, MQ, NH, NW, NZ, OU, PE, QF, S4, SQ, TP, UA, US, VS, WE, WN
2019: CX BOS-HKG, WE HKG-HKT, CA HKT-PEK-EWR, B6 EWR-BOS