But to answer your question, my understanding is that most nodes drop old pending txs after 3 days.
... and some other nodes resend pending txs after 4 days 
for example, go to https://blockchain.info/rejected
take the link with this comment "The Maximum Very Small Inputs Exceeded. Expected at least (150000) satoshi fee"
[copy] txid for a transaction which was rejected by bc.i today
then go to https://tradeblock.com/bitcoin/
[paste] the txid and get the original date when this transaction first time appeared on the network
have fun.
Nodes do not continue to push txs that have already been propagated throughout the network.
They all pass it on until the whole node network is aware of that specific tx.
What you are pointing to are either custom programs that are either broken or being malicious,
and are purposefully pushing txs that are either invalid, too low of a fee, doublespent, or spam.
For the average user, their bitcoins will "return" to their wallets after 3 days.