1. OP departed AMS, so this has nothing to do with TSA.
2. OP would have reclaimed his bags at SEA for CBP processing, before dropping them for the onward segment to PDX. Presumably, if he goes to the trouble of locking his bags, he would have seen that there were no locks. This eliminates the possibility that TSA at SEA cut off the locks.
Bottom line here is that the locks were cut off by the air carrier, a security contractor or AMS security. Whatever notes it is that TSA sometimes leave and its practices, have nothing to do with OP's complaint.
Practically speaking, locking a suitcase is futile. It does not require a key to open a lock. All it takes is a lock cutter and 10 seconds. By locking a bag, all you do is signal to thieves that your bag contains items worth stealing (which it should not). Best to not lock your bag because anybody who wants to open the bag will open it, whether they have a legal or illegal purpose.
As to having someone stuff contraband into your case, that too can be done by anyone with a $3 lock cutter who wants to do harm to you. But, a better question is why anyone would do this to a random stranger.