Are travel agencies scamming people with flights from Europe transiting through US?

Westin

All credit to Trip.com and United for offering you a credit.

I would suggest you check the terms of the UA travel credit particularly for the expiration date.  

There should be be plenty of options between Madrid and Mexico direct. Probably more than if you were travelling from the U.K.  At the current time with regularly changing travel restrictions I think a direct service flight would be most preferable and safer.

epm-84

Tigerchaser said:

Im travelling from Spain, flight was booked with trip.com

For future reference trip.com are based in Singapore, so they may not always follow the procedure you would expect from a British company.