Indeed, many ISP's put hard data caps on data usage and punish (if not throttle) people that are abusive. In the age of Netflix, figuring out what is what can be hard but drilling down to the network traffic in question can clarify that. However, this even pertains to network managers/engineers at businesses. Whether it be for the data management/hassle avoidance items mentioned above and/or because employees are engaging in unethical (if not illegal) things at work, all of the above needs to be done. General training on virus/malware avoidance has to be engaged in but users should not be expected to get it all right themselves as they will often make very poor choices. Technologies such as firewalls, internet filters and overall limits on what can and cannot be done with emails (like on an Exchange Server or via the Web) have to be implemented. Blocking of certain tactics, linkages to certain sites, and general spam filtering will weed out a good many of the problems that can exist. However, end-user education can go a long way as well...

People that are constantly running into virus/email problems need to be trained or let go. People that use email to engage in cyber-stalking or other illegal/illicit behavior need to severely punished if not fired. Users that intentionally use the email address of others for surreptitious means should also be dispensed with because of what can truly go wrong if/when that is allowed to occur. Coupling email controls and policies with things such as idle-timer lockouts for applications and Windows at large is also a good idea as doing the latter can prevent naughty emails.
Conclusion

In short, email sender concealment can be a sign of bad deeds but people should not jump the gun and act out of ignorance. if, however, it is clear that concealment is a means to enable illegal (or even perhaps unethical) behavior, then it might need to be shut down (especially if it's illegal). There is a time and a place where this tactic can and should be used and is not illegal but the opposite clearly exists as well.